Add more structure to errors (continuation of #34684) (#42640)

* Add structured errors in Animations, TabView, ChangeNotifier

* Add structured error on MaterialPageRoute, BoxBorder, DecorationImagePainter, TextSpan

* Add structured errors in Debug

* Fix test errors

* Add structured errors in Scaffold and Stepper

* Add structured errors in part of Rendering Layer

* Fix failing test due to FloatingPoint precision

* Fix failing tests due to precision error and not using final

* Fix failing test due to floating precision error with RegEx instead

* Add structured error in CustomLayout and increase test coverage

* Add structured error & its test in ListBody

* Add structured error in ProxyBox and increase test coverage

* Add structured error message in Viewport

* Fix styles and add more assertions on ErrorHint and DiagnosticProperty

* Add structured error in scheduler/binding and scheduler/ticker
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured error in AssetBundle and TextInput
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured errors in several widgets #1
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Remove unused import
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add assertions on hint messages
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Fix catch spacing
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured error in several widgets part 2 and increase code coverage
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured error in flutter_test/widget_tester

* Fix floating precision accuracy by using RegExp
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Remove todo to add tests in Scaffold showBottomSheet
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Fix reviews by indenting lines and fixing the assertion orders
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Fix failing tests due to renaming class
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Try skipping the NetworkBundleTest
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Remove leading space in material/debug error hint
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

packages/flutter/lib/src/animation/animations.dart

@@ -439,12 +439,14 @@ class CurvedAnimation extends Animation<double> with AnimationWithParentMixin<do
         final double transformedValue = activeCurve.transform(t);
         final double roundedTransformedValue = transformedValue.round().toDouble();
         if (roundedTransformedValue != t) {
-          throw FlutterError(
-            'Invalid curve endpoint at $t.\n'
-            'Curves must map 0.0 to near zero and 1.0 to near one but '
-            '${activeCurve.runtimeType} mapped $t to $transformedValue, which '
-            'is near $roundedTransformedValue.'
-          );
+          throw FlutterError.fromParts(<DiagnosticsNode>[
+            ErrorSummary('Invalid curve endpoint at $t.'),
+            ErrorDescription(
+              'Curves must map 0.0 to near zero and 1.0 to near one but '
+              '${activeCurve.runtimeType} mapped $t to $transformedValue, which '
+              'is near $roundedTransformedValue.'
+            )
+          ]);
         }
         return true;
       }());

packages/flutter/lib/src/cupertino/tab_view.dart

@@ -192,29 +192,33 @@ class _CupertinoTabViewState extends State<CupertinoTabView> {
   Route<dynamic> _onUnknownRoute(RouteSettings settings) {
     assert(() {
       if (widget.onUnknownRoute == null) {
-        throw FlutterError(
-          'Could not find a generator for route $settings in the $runtimeType.\n'
-          'Generators for routes are searched for in the following order:\n'
-          ' 1. For the "/" route, the "builder" property, if non-null, is used.\n'
-          ' 2. Otherwise, the "routes" table is used, if it has an entry for '
-          'the route.\n'
-          ' 3. Otherwise, onGenerateRoute is called. It should return a '
-          'non-null value for any valid route not handled by "builder" and "routes".\n'
-          ' 4. Finally if all else fails onUnknownRoute is called.\n'
-          'Unfortunately, onUnknownRoute was not set.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('Could not find a generator for route $settings in the $runtimeType.'),
+          ErrorDescription(
+            'Generators for routes are searched for in the following order:\n'
+            ' 1. For the "/" route, the "builder" property, if non-null, is used.\n'
+            ' 2. Otherwise, the "routes" table is used, if it has an entry for '
+            'the route.\n'
+            ' 3. Otherwise, onGenerateRoute is called. It should return a '
+            'non-null value for any valid route not handled by "builder" and "routes".\n'
+            ' 4. Finally if all else fails onUnknownRoute is called.\n'
+            'Unfortunately, onUnknownRoute was not set.'
+          )
+        ]);
       }
       return true;
     }());
     final Route<dynamic> result = widget.onUnknownRoute(settings);
     assert(() {
       if (result == null) {
-        throw FlutterError(
-          'The onUnknownRoute callback returned null.\n'
-          'When the $runtimeType requested the route $settings from its '
-          'onUnknownRoute callback, the callback returned null. Such callbacks '
-          'must never return null.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('The onUnknownRoute callback returned null.'),
+          ErrorDescription(
+            'When the $runtimeType requested the route $settings from its '
+            'onUnknownRoute callback, the callback returned null. Such callbacks '
+            'must never return null.'
+          )
+        ]);
       }
       return true;
     }());

packages/flutter/lib/src/foundation/change_notifier.dart

@@ -102,10 +102,10 @@ class ChangeNotifier implements Listenable {
   bool _debugAssertNotDisposed() {
     assert(() {
       if (_listeners == null) {
-        throw FlutterError(
-          'A $runtimeType was used after being disposed.\n'
-          'Once you have called dispose() on a $runtimeType, it can no longer be used.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('A $runtimeType was used after being disposed.'),
+          ErrorDescription('Once you have called dispose() on a $runtimeType, it can no longer be used.')
+        ]);
       }
       return true;
     }());

packages/flutter/lib/src/material/debug.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
 import 'material.dart';
@@ -24,45 +25,26 @@ import 'scaffold.dart' show Scaffold;
 bool debugCheckHasMaterial(BuildContext context) {
   assert(() {
     if (context.widget is! Material && context.ancestorWidgetOfExactType(Material) == null) {
-      final StringBuffer message = StringBuffer();
-      message.writeln('No Material widget found.');
-      message.writeln(
-        '${context.widget.runtimeType} widgets require a Material '
-        'widget ancestor.'
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('No Material widget found.'),
+        ErrorDescription(
+          '${context.widget.runtimeType} widgets require a Material '
+          'widget ancestor.\n'
+          'In material design, most widgets are conceptually "printed" on '
+          'a sheet of material. In Flutter\'s material library, that '
+          'material is represented by the Material widget. It is the '
+          'Material widget that renders ink splashes, for instance. '
+          'Because of this, many material library widgets require that '
+          'there be a Material widget in the tree above them.'
+        ),
+        ErrorHint(
+          'To introduce a Material widget, you can either directly '
+          'include one, or use a widget that contains Material itself, '
+          'such as a Card, Dialog, Drawer, or Scaffold.',
+        ),
+        ...context.describeMissingAncestor(expectedAncestorType: Material)
+      ]
       );
-      message.writeln(
-        'In material design, most widgets are conceptually "printed" on '
-        'a sheet of material. In Flutter\'s material library, that '
-        'material is represented by the Material widget. It is the '
-        'Material widget that renders ink splashes, for instance. '
-        'Because of this, many material library widgets require that '
-        'there be a Material widget in the tree above them.'
-      );
-      message.writeln(
-        'To introduce a Material widget, you can either directly '
-        'include one, or use a widget that contains Material itself, '
-        'such as a Card, Dialog, Drawer, or Scaffold.'
-      );
-      message.writeln(
-        'The specific widget that could not find a Material ancestor was:'
-      );
-      message.writeln('  ${context.widget}');
-      final List<Widget> ancestors = <Widget>[];
-      context.visitAncestorElements((Element element) {
-        ancestors.add(element.widget);
-        return true;
-      });
-      if (ancestors.isNotEmpty) {
-        message.write('The ancestors of this widget were:');
-        for (Widget ancestor in ancestors)
-          message.write('\n  $ancestor');
-      } else {
-        message.writeln(
-          'This widget is the root of the tree, so it has no '
-          'ancestors, let alone a "Material" ancestor.'
-        );
-      }
-      throw FlutterError(message.toString());
     }
     return true;
   }());
@@ -87,42 +69,24 @@ bool debugCheckHasMaterial(BuildContext context) {
 bool debugCheckHasMaterialLocalizations(BuildContext context) {
   assert(() {
     if (Localizations.of<MaterialLocalizations>(context, MaterialLocalizations) == null) {
-      final StringBuffer message = StringBuffer();
-      message.writeln('No MaterialLocalizations found.');
-      message.writeln(
-        '${context.widget.runtimeType} widgets require MaterialLocalizations '
-        'to be provided by a Localizations widget ancestor.'
-      );
-      message.writeln(
-        'Localizations are used to generate many different messages, labels,'
-        'and abbreviations which are used by the material library. '
-      );
-      message.writeln(
-        'To introduce a MaterialLocalizations, either use a '
-        ' MaterialApp at the root of your application to include them '
-        'automatically, or add a Localization widget with a '
-        'MaterialLocalizations delegate.'
-      );
-      message.writeln(
-        'The specific widget that could not find a MaterialLocalizations ancestor was:'
-      );
-      message.writeln('  ${context.widget}');
-      final List<Widget> ancestors = <Widget>[];
-      context.visitAncestorElements((Element element) {
-        ancestors.add(element.widget);
-        return true;
-      });
-      if (ancestors.isNotEmpty) {
-        message.write('The ancestors of this widget were:');
-        for (Widget ancestor in ancestors)
-          message.write('\n  $ancestor');
-      } else {
-        message.writeln(
-          'This widget is the root of the tree, so it has no '
-          'ancestors, let alone a "Localizations" ancestor.'
-        );
-      }
-      throw FlutterError(message.toString());
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('No MaterialLocalizations found.'),
+        ErrorDescription(
+          '${context.widget.runtimeType} widgets require MaterialLocalizations '
+          'to be provided by a Localizations widget ancestor.'
+        ),
+        ErrorDescription(
+          'Localizations are used to generate many different messages, labels,'
+          'and abbreviations which are used by the material library.'
+        ),
+        ErrorHint(
+          'To introduce a MaterialLocalizations, either use a '
+          'MaterialApp at the root of your application to include them '
+          'automatically, or add a Localization widget with a '
+          'MaterialLocalizations delegate.'
+        ),
+        ...context.describeMissingAncestor(expectedAncestorType: MaterialLocalizations)
+      ]);
     }
     return true;
   }());
@@ -145,17 +109,15 @@ bool debugCheckHasMaterialLocalizations(BuildContext context) {
 bool debugCheckHasScaffold(BuildContext context) {
   assert(() {
     if (context.widget is! Scaffold && context.ancestorWidgetOfExactType(Scaffold) == null) {
-      final Element element = context;
-      throw FlutterError(
-          'No Scaffold widget found.\n'
-          '${context.widget.runtimeType} widgets require a Scaffold widget ancestor.\n'
-          'The Specific widget that could not find a Scaffold ancestor was:\n'
-          '  ${context.widget}\n'
-          'The ownership chain for the affected widget is:\n'
-          '  ${element.debugGetCreatorChain(10)}\n'
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('No Scaffold widget found.'),
+        ErrorDescription('${context.widget.runtimeType} widgets require a Scaffold widget ancestor.'),
+        ...context.describeMissingAncestor(expectedAncestorType: Scaffold),
+        ErrorHint(
           'Typically, the Scaffold widget is introduced by the MaterialApp or '
           'WidgetsApp widget at the top of your application widget tree.'
-      );
+        )
+      ]);
     }
     return true;
   }());

packages/flutter/lib/src/material/page.dart

@@ -87,10 +87,10 @@ class MaterialPageRoute<T> extends PageRoute<T> {
     final Widget result = builder(context);
     assert(() {
       if (result == null) {
-        throw FlutterError(
-          'The builder for route "${settings.name}" returned null.\n'
-          'Route builders must never return null.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('The builder for route "${settings.name}" returned null.'),
+          ErrorDescription('Route builders must never return null.')
+        ]);
       }
       return true;
     }());

packages/flutter/lib/src/material/scaffold.dart

@@ -1310,25 +1310,32 @@ class Scaffold extends StatefulWidget {
     final ScaffoldState result = context.ancestorStateOfType(const TypeMatcher<ScaffoldState>());
     if (nullOk || result != null)
       return result;
-    throw FlutterError(
-      'Scaffold.of() called with a context that does not contain a Scaffold.\n'
-      'No Scaffold ancestor could be found starting from the context that was passed to Scaffold.of(). '
-      'This usually happens when the context provided is from the same StatefulWidget as that '
-      'whose build function actually creates the Scaffold widget being sought.\n'
-      'There are several ways to avoid this problem. The simplest is to use a Builder to get a '
-      'context that is "under" the Scaffold. For an example of this, please see the '
-      'documentation for Scaffold.of():\n'
-      '  https://api.flutter.dev/flutter/material/Scaffold/of.html\n'
-      'A more efficient solution is to split your build function into several widgets. This '
-      'introduces a new context from which you can obtain the Scaffold. In this solution, '
-      'you would have an outer widget that creates the Scaffold populated by instances of '
-      'your new inner widgets, and then in these inner widgets you would use Scaffold.of().\n'
-      'A less elegant but more expedient solution is assign a GlobalKey to the Scaffold, '
-      'then use the key.currentState property to obtain the ScaffoldState rather than '
-      'using the Scaffold.of() function.\n'
-      'The context used was:\n'
-      '  $context'
-    );
+    throw FlutterError.fromParts(<DiagnosticsNode>[
+      ErrorSummary(
+        'Scaffold.of() called with a context that does not contain a Scaffold.'
+      ),
+      ErrorDescription(
+        'No Scaffold ancestor could be found starting from the context that was passed to Scaffold.of(). '
+        'This usually happens when the context provided is from the same StatefulWidget as that '
+        'whose build function actually creates the Scaffold widget being sought.'
+      ),
+      ErrorHint(
+        'There are several ways to avoid this problem. The simplest is to use a Builder to get a '
+        'context that is "under" the Scaffold. For an example of this, please see the '
+        'documentation for Scaffold.of():\n'
+        '  https://api.flutter.dev/flutter/material/Scaffold/of.html'
+      ),
+      ErrorHint(
+        'A more efficient solution is to split your build function into several widgets. This '
+        'introduces a new context from which you can obtain the Scaffold. In this solution, '
+        'you would have an outer widget that creates the Scaffold populated by instances of '
+        'your new inner widgets, and then in these inner widgets you would use Scaffold.of().\n'
+        'A less elegant but more expedient solution is assign a GlobalKey to the Scaffold, '
+        'then use the key.currentState property to obtain the ScaffoldState rather than '
+        'using the Scaffold.of() function.'
+      ),
+      context.describeElement('The context used was')
+    ]);
   }
 
   /// Returns a [ValueListenable] for the [ScaffoldGeometry] for the closest
@@ -1354,22 +1361,28 @@ class Scaffold extends StatefulWidget {
   static ValueListenable<ScaffoldGeometry> geometryOf(BuildContext context) {
     final _ScaffoldScope scaffoldScope = context.inheritFromWidgetOfExactType(_ScaffoldScope);
     if (scaffoldScope == null)
-      throw FlutterError(
-        'Scaffold.geometryOf() called with a context that does not contain a Scaffold.\n'
-        'This usually happens when the context provided is from the same StatefulWidget as that '
-        'whose build function actually creates the Scaffold widget being sought.\n'
-        'There are several ways to avoid this problem. The simplest is to use a Builder to get a '
-        'context that is "under" the Scaffold. For an example of this, please see the '
-        'documentation for Scaffold.of():\n'
-        '  https://api.flutter.dev/flutter/material/Scaffold/of.html\n'
-        'A more efficient solution is to split your build function into several widgets. This '
-        'introduces a new context from which you can obtain the Scaffold. In this solution, '
-        'you would have an outer widget that creates the Scaffold populated by instances of '
-        'your new inner widgets, and then in these inner widgets you would use Scaffold.geometryOf().\n'
-        'The context used was:\n'
-        '  $context'
-      );
-
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary(
+          'Scaffold.geometryOf() called with a context that does not contain a Scaffold.'
+        ),
+        ErrorDescription(
+          'This usually happens when the context provided is from the same StatefulWidget as that '
+          'whose build function actually creates the Scaffold widget being sought.'
+        ),
+        ErrorHint(
+          'There are several ways to avoid this problem. The simplest is to use a Builder to get a '
+          'context that is "under" the Scaffold. For an example of this, please see the '
+          'documentation for Scaffold.of():\n'
+          '  https://api.flutter.dev/flutter/material/Scaffold/of.html'
+        ),
+        ErrorHint(
+          'A more efficient solution is to split your build function into several widgets. This '
+          'introduces a new context from which you can obtain the Scaffold. In this solution, '
+          'you would have an outer widget that creates the Scaffold populated by instances of '
+          'your new inner widgets, and then in these inner widgets you would use Scaffold.geometryOf().',
+        ),
+        context.describeElement('The context used was')
+      ]);
     return scaffoldScope.geometryNotifier;
   }
 
@@ -1679,9 +1692,9 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     assert(() {
       if (widget.bottomSheet != null && isPersistent && _currentBottomSheet != null) {
         throw FlutterError(
-          'Scaffold.bottomSheet cannot be specified while a bottom sheet displayed '
-          'with showBottomSheet() is still visible.\n Rebuild the Scaffold with a null '
-          'bottomSheet before calling showBottomSheet().'
+          'Scaffold.bottomSheet cannot be specified while a bottom sheet'
+          'displayed with showBottomSheet() is still visible.\n'
+          'Rebuild the Scaffold with a null bottomSheet before calling showBottomSheet().'
         );
       }
       return true;
@@ -1818,9 +1831,9 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     assert(() {
       if (widget.bottomSheet != null) {
         throw FlutterError(
-          'Scaffold.bottomSheet cannot be specified while a bottom sheet displayed '
-          'with showBottomSheet() is still visible.\n Rebuild the Scaffold with a null '
-          'bottomSheet before calling showBottomSheet().'
+          'Scaffold.bottomSheet cannot be specified while a bottom sheet'
+          'displayed with showBottomSheet() is still visible.\n'
+          'Rebuild the Scaffold with a null bottomSheet before calling showBottomSheet().'
         );
       }
       return true;
@@ -1952,12 +1965,17 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     if (widget.bottomSheet != oldWidget.bottomSheet) {
       assert(() {
         if (widget.bottomSheet != null && _currentBottomSheet?._isLocalHistoryEntry == true) {
-          throw FlutterError(
-            'Scaffold.bottomSheet cannot be specified while a bottom sheet displayed '
-            'with showBottomSheet() is still visible.\n Use the PersistentBottomSheetController '
-            'returned by showBottomSheet() to close the old bottom sheet before creating '
-            'a Scaffold with a (non null) bottomSheet.'
-          );
+          throw FlutterError.fromParts(<DiagnosticsNode>[
+            ErrorSummary(
+              'Scaffold.bottomSheet cannot be specified while a bottom sheet displayed '
+              'with showBottomSheet() is still visible.'
+            ),
+            ErrorHint(
+              'Use the PersistentBottomSheetController '
+              'returned by showBottomSheet() to close the old bottom sheet before creating '
+              'a Scaffold with a (non null) bottomSheet.'
+            ),
+          ]);
         }
         return true;
       }());

packages/flutter/lib/src/material/stepper.dart

@@ -672,8 +672,9 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
     assert(() {
       if (context.ancestorWidgetOfExactType(Stepper) != null)
         throw FlutterError(
-          'Steppers must not be nested. The material specification advises '
-          'that one should avoid embedding steppers within steppers. '
+          'Steppers must not be nested.\n'
+          'The material specification advises that one should avoid embedding '
+          'steppers within steppers. '
           'https://material.io/archive/guidelines/components/steppers.html#steppers-usage'
         );
       return true;

packages/flutter/lib/src/painting/box_border.dart

@@ -152,14 +152,16 @@ abstract class BoxBorder extends ShapeBorder {
         bottom: BorderSide.lerp(a.bottom, b.bottom, t),
       );
     }
-    throw FlutterError(
-      'BoxBorder.lerp can only interpolate Border and BorderDirectional classes.\n'
-      'BoxBorder.lerp() was called with two objects of type ${a.runtimeType} and ${b.runtimeType}:\n'
-      '  $a\n'
-      '  $b\n'
-      'However, only Border and BorderDirectional classes are supported by this method. '
-      'For a more general interpolation method, consider using ShapeBorder.lerp instead.'
-    );
+    throw FlutterError.fromParts(<DiagnosticsNode>[
+      ErrorSummary('BoxBorder.lerp can only interpolate Border and BorderDirectional classes.'),
+      ErrorDescription(
+        'BoxBorder.lerp() was called with two objects of type ${a.runtimeType} and ${b.runtimeType}:\n'
+        '  $a\n'
+        '  $b\n'
+        'However, only Border and BorderDirectional classes are supported by this method.'
+      ),
+      ErrorHint('For a more general interpolation method, consider using ShapeBorder.lerp instead.'),
+    ]);
   }
 
   @override

packages/flutter/lib/src/painting/decoration_image.dart

@@ -221,15 +221,15 @@ class DecorationImagePainter {
         // We check this first so that the assert will fire immediately, not just
         // when the image is ready.
         if (configuration.textDirection == null) {
-          throw FlutterError(
-            'ImageDecoration.matchTextDirection can only be used when a TextDirection is available.\n'
-            'When DecorationImagePainter.paint() was called, there was no text direction provided '
-            'in the ImageConfiguration object to match.\n'
-            'The DecorationImage was:\n'
-            '  $_details\n'
-            'The ImageConfiguration was:\n'
-            '  $configuration'
-          );
+          throw FlutterError.fromParts(<DiagnosticsNode>[
+            ErrorSummary('ImageDecoration.matchTextDirection can only be used when a TextDirection is available.'),
+            ErrorDescription(
+              'When DecorationImagePainter.paint() was called, there was no text direction provided '
+              'in the ImageConfiguration object to match.'
+            ),
+            DiagnosticsProperty<DecorationImage>('The DecorationImage was', _details, style: DiagnosticsTreeStyle.errorProperty),
+            DiagnosticsProperty<ImageConfiguration>('The ImageConfiguration was', configuration, style: DiagnosticsTreeStyle.errorProperty),
+          ]);
         }
         return true;
       }());

packages/flutter/lib/src/painting/text_span.dart

@@ -362,12 +362,15 @@ class TextSpan extends InlineSpan {
     assert(() {
       if (children != null) {
         for (InlineSpan child in children) {
-          assert(child != null,
-            'TextSpan contains a null child.\n...'
-            'A TextSpan object with a non-null child list should not have any nulls in its child list.\n'
-            'The full text in question was:\n'
-            '${toStringDeep(prefixLineOne: '  ')}'
-          );
+          if (child == null) {
+            throw FlutterError.fromParts(<DiagnosticsNode>[
+              ErrorSummary('TextSpan contains a null child.'),
+              ErrorDescription(
+                  'A TextSpan object with a non-null child list should not have any nulls in its child list.'),
+              toDiagnosticsNode(name: 'The full text in question was',
+                  style: DiagnosticsTreeStyle.errorProperty),
+            ]);
+          }
           assert(child.debugAssertIsValid());
         }
       }

packages/flutter/lib/src/rendering/custom_layout.dart

@@ -134,27 +134,29 @@ abstract class MultiChildLayoutDelegate {
     final RenderBox child = _idToChild[childId];
     assert(() {
       if (child == null) {
-        throw FlutterError(
-          'The $this custom multichild layout delegate tried to lay out a non-existent child.\n'
-          'There is no child with the id "$childId".'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('The $this custom multichild layout delegate tried to lay out a non-existent child.'),
+          ErrorDescription('There is no child with the id "$childId".')
+        ]);
       }
       if (!_debugChildrenNeedingLayout.remove(child)) {
-        throw FlutterError(
-          'The $this custom multichild layout delegate tried to lay out the child with id "$childId" more than once.\n'
-          'Each child must be laid out exactly once.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('The $this custom multichild layout delegate tried to lay out the child with id "$childId" more than once.'),
+          ErrorDescription('Each child must be laid out exactly once.')
+        ]);
       }
       try {
         assert(constraints.debugAssertIsValid(isAppliedConstraint: true));
       } on AssertionError catch (exception) {
-        throw FlutterError(
-          'The $this custom multichild layout delegate provided invalid box constraints for the child with id "$childId".\n'
-          '$exception\n'
-          'The minimum width and height must be greater than or equal to zero.\n'
-          'The maximum width must be greater than or equal to the minimum width.\n'
-          'The maximum height must be greater than or equal to the minimum height.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('The $this custom multichild layout delegate provided invalid box constraints for the child with id "$childId".'),
+          DiagnosticsProperty<AssertionError>('Exception', exception, showName: false),
+          ErrorDescription(
+            'The minimum width and height must be greater than or equal to zero.\n'
+            'The maximum width must be greater than or equal to the minimum width.\n'
+            'The maximum height must be greater than or equal to the minimum height.'
+          )
+        ]);
       }
       return true;
     }());
@@ -172,15 +174,15 @@ abstract class MultiChildLayoutDelegate {
     final RenderBox child = _idToChild[childId];
     assert(() {
       if (child == null) {
-        throw FlutterError(
-          'The $this custom multichild layout delegate tried to position out a non-existent child:\n'
-          'There is no child with the id "$childId".'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('The $this custom multichild layout delegate tried to position out a non-existent child:'),
+          ErrorDescription('There is no child with the id "$childId".')
+        ]);
       }
       if (offset == null) {
-        throw FlutterError(
-          'The $this custom multichild layout delegate provided a null position for the child with id "$childId".'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('The $this custom multichild layout delegate provided a null position for the child with id "$childId".')
+        ]);
       }
       return true;
     }());
@@ -188,9 +190,9 @@ abstract class MultiChildLayoutDelegate {
     childParentData.offset = offset;
   }
 
-  String _debugDescribeChild(RenderBox child) {
+  DiagnosticsNode _debugDescribeChild(RenderBox child) {
     final MultiChildLayoutParentData childParentData = child.parentData;
-    return '${childParentData.id}: $child';
+    return DiagnosticsProperty<RenderBox>('${childParentData.id}', child);
   }
 
   void _callPerformLayout(Size size, RenderBox firstChild) {
@@ -213,11 +215,10 @@ abstract class MultiChildLayoutDelegate {
         final MultiChildLayoutParentData childParentData = child.parentData;
         assert(() {
           if (childParentData.id == null) {
-            throw FlutterError(
-              'The following child has no ID:\n'
-              '  $child\n'
-              'Every child of a RenderCustomMultiChildLayoutBox must have an ID in its parent data.'
-            );
+            throw FlutterError.fromParts(<DiagnosticsNode>[
+              ErrorSummary('Every child of a RenderCustomMultiChildLayoutBox must have an ID in its parent data.'),
+              child.describeForError('The following child has no ID'),
+            ]);
           }
           return true;
         }());
@@ -231,19 +232,17 @@ abstract class MultiChildLayoutDelegate {
       performLayout(size);
       assert(() {
         if (_debugChildrenNeedingLayout.isNotEmpty) {
-          if (_debugChildrenNeedingLayout.length > 1) {
-            throw FlutterError(
-              'The $this custom multichild layout delegate forgot to lay out the following children:\n'
-              '  ${_debugChildrenNeedingLayout.map<String>(_debugDescribeChild).join("\n  ")}\n'
-              'Each child must be laid out exactly once.'
-            );
-          } else {
-            throw FlutterError(
-              'The $this custom multichild layout delegate forgot to lay out the following child:\n'
-              '  ${_debugDescribeChild(_debugChildrenNeedingLayout.single)}\n'
-              'Each child must be laid out exactly once.'
-            );
-          }
+          throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('Each child must be laid out exactly once.'),
+            DiagnosticsBlock(
+              name:
+                'The $this custom multichild layout delegate forgot '
+                'to lay out the following '
+                '${_debugChildrenNeedingLayout.length > 1 ? 'children' : 'child'}',
+              properties: _debugChildrenNeedingLayout.map<DiagnosticsNode>(_debugDescribeChild).toList(),
+              style: DiagnosticsTreeStyle.whitespace,
+            ),
+          ]);
         }
         return true;
       }());

packages/flutter/lib/src/rendering/custom_paint.dart

@@ -539,24 +539,27 @@ class RenderCustomPaint extends RenderProxyBox {
       // below that number.
       final int debugNewCanvasSaveCount = canvas.getSaveCount();
       if (debugNewCanvasSaveCount > debugPreviousCanvasSaveCount) {
-        throw FlutterError(
-          'The $painter custom painter called canvas.save() or canvas.saveLayer() at least '
-          '${debugNewCanvasSaveCount - debugPreviousCanvasSaveCount} more '
-          'time${debugNewCanvasSaveCount - debugPreviousCanvasSaveCount == 1 ? '' : 's' } '
-          'than it called canvas.restore().\n'
-          'This leaves the canvas in an inconsistent state and will probably result in a broken display.\n'
-          'You must pair each call to save()/saveLayer() with a later matching call to restore().'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary(
+            'The $painter custom painter called canvas.save() or canvas.saveLayer() at least '
+            '${debugNewCanvasSaveCount - debugPreviousCanvasSaveCount} more '
+            'time${debugNewCanvasSaveCount - debugPreviousCanvasSaveCount == 1 ? '' : 's' } '
+            'than it called canvas.restore().'
+          ),
+          ErrorDescription('This leaves the canvas in an inconsistent state and will probably result in a broken display.'),
+          ErrorHint('You must pair each call to save()/saveLayer() with a later matching call to restore().')
+        ]);
       }
       if (debugNewCanvasSaveCount < debugPreviousCanvasSaveCount) {
-        throw FlutterError(
-          'The $painter custom painter called canvas.restore() '
-          '${debugPreviousCanvasSaveCount - debugNewCanvasSaveCount} more '
-          'time${debugPreviousCanvasSaveCount - debugNewCanvasSaveCount == 1 ? '' : 's' } '
-          'than it called canvas.save() or canvas.saveLayer().\n'
-          'This leaves the canvas in an inconsistent state and will result in a broken display.\n'
-          'You should only call restore() if you first called save() or saveLayer().'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('The $painter custom painter called canvas.restore() '
+            '${debugPreviousCanvasSaveCount - debugNewCanvasSaveCount} more '
+            'time${debugPreviousCanvasSaveCount - debugNewCanvasSaveCount == 1 ? '' : 's' } '
+            'than it called canvas.save() or canvas.saveLayer().'
+          ),
+          ErrorDescription('This leaves the canvas in an inconsistent state and will result in a broken display.'),
+          ErrorHint('You should only call restore() if you first called save() or saveLayer().')
+        ]);
       }
       return debugNewCanvasSaveCount == debugPreviousCanvasSaveCount;
     }());
@@ -611,10 +614,12 @@ class RenderCustomPaint extends RenderProxyBox {
   ) {
     assert(() {
       if (child == null && children.isNotEmpty) {
-        throw FlutterError(
-          '$runtimeType does not have a child widget but received a non-empty list of child SemanticsNode:\n'
-          '${children.join('\n')}'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary(
+            '$runtimeType does not have a child widget but received a non-empty list of child SemanticsNode:\n'
+            '${children.join('\n')}'
+          )
+        ]);
       }
       return true;
     }());
@@ -677,24 +682,20 @@ class RenderCustomPaint extends RenderProxyBox {
 
     assert(() {
       final Map<Key, int> keys = HashMap<Key, int>();
-      final StringBuffer errors = StringBuffer();
+      final List<DiagnosticsNode> information = <DiagnosticsNode>[];
       for (int i = 0; i < newChildSemantics.length; i += 1) {
         final CustomPainterSemantics child = newChildSemantics[i];
         if (child.key != null) {
           if (keys.containsKey(child.key)) {
-            errors.writeln(
-              '- duplicate key ${child.key} found at position $i',
-            );
+            information.add(ErrorDescription('- duplicate key ${child.key} found at position $i'));
           }
           keys[child.key] = i;
         }
       }
 
-      if (errors.isNotEmpty) {
-        throw FlutterError(
-          'Failed to update the list of CustomPainterSemantics:\n'
-          '$errors'
-        );
+      if (information.isNotEmpty) {
+        information.insert(0, ErrorSummary('Failed to update the list of CustomPainterSemantics:'));
+        throw FlutterError.fromParts(information);
       }
 
       return true;

packages/flutter/lib/src/rendering/flow.dart

@@ -319,11 +319,13 @@ class RenderFlow extends RenderBox
     final FlowParentData childParentData = child.parentData;
     assert(() {
       if (childParentData._transform != null) {
-        throw FlutterError(
-          'Cannot call paintChild twice for the same child.\n'
-          'The flow delegate of type ${_delegate.runtimeType} attempted to '
-          'paint child $i multiple times, which is not permitted.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('Cannot call paintChild twice for the same child.'),
+          ErrorDescription(
+            'The flow delegate of type ${_delegate.runtimeType} attempted to '
+            'paint child $i multiple times, which is not permitted.'
+          )
+        ]);
       }
       return true;
     }());

packages/flutter/lib/src/rendering/list_body.dart

@@ -74,13 +74,18 @@ class RenderListBody extends RenderBox
             return true;
           break;
       }
-      throw FlutterError(
-        'RenderListBody must have unlimited space along its main axis.\n'
-        'RenderListBody does not clip or resize its children, so it must be '
-        'placed in a parent that does not constrain the main '
-        'axis. You probably want to put the RenderListBody inside a '
-        'RenderViewport with a matching main axis.'
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('RenderListBody must have unlimited space along its main axis.'),
+        ErrorDescription(
+          'RenderListBody does not clip or resize its children, so it must be '
+          'placed in a parent that does not constrain the main '
+          'axis.'
+        ),
+        ErrorHint(
+          'You probably want to put the RenderListBody inside a '
+          'RenderViewport with a matching main axis.'
+        )
+      ]);
     }());
     assert(() {
       switch (mainAxis) {
@@ -96,16 +101,23 @@ class RenderListBody extends RenderBox
       // TODO(ianh): Detect if we're actually nested blocks and say something
       // more specific to the exact situation in that case, and don't mention
       // nesting blocks in the negative case.
-      throw FlutterError(
-        'RenderListBody must have a bounded constraint for its cross axis.\n'
-        'RenderListBody forces its children to expand to fit the RenderListBody\'s container, '
-        'so it must be placed in a parent that constrains the cross '
-        'axis to a finite dimension. If you are attempting to nest a RenderListBody with '
-        'one direction inside one of another direction, you will want to '
-        'wrap the inner one inside a box that fixes the dimension in that direction, '
-        'for example, a RenderIntrinsicWidth or RenderIntrinsicHeight object. '
-        'This is relatively expensive, however.' // (that's why we don't do it automatically)
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('RenderListBody must have a bounded constraint for its cross axis.'),
+        ErrorDescription(
+          'RenderListBody forces its children to expand to fit the RenderListBody\'s container, '
+          'so it must be placed in a parent that constrains the cross '
+          'axis to a finite dimension.'
+        ),
+        // TODO(jacobr): this hint is a great candidate to promote to being an
+        // automated quick fix in the future.
+        ErrorHint(
+          'If you are attempting to nest a RenderListBody with '
+          'one direction inside one of another direction, you will want to '
+          'wrap the inner one inside a box that fixes the dimension in that direction, '
+          'for example, a RenderIntrinsicWidth or RenderIntrinsicHeight object. '
+          'This is relatively expensive, however.' // (that's why we don't do it automatically)
+        )
+      ]);
     }());
     double mainAxisExtent = 0.0;
     RenderBox child = firstChild;

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -456,13 +456,15 @@ class RenderAspectRatio extends RenderProxyBox {
     assert(constraints.debugAssertIsValid());
     assert(() {
       if (!constraints.hasBoundedWidth && !constraints.hasBoundedHeight) {
-        throw FlutterError(
-          '$runtimeType has unbounded constraints.\n'
-          'This $runtimeType was given an aspect ratio of $aspectRatio but was given '
-          'both unbounded width and unbounded height constraints. Because both '
-          'constraints were unbounded, this render object doesn\'t know how much '
-          'size to consume.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('$runtimeType has unbounded constraints.'),
+          ErrorDescription(
+            'This $runtimeType was given an aspect ratio of $aspectRatio but was given '
+            'both unbounded width and unbounded height constraints. Because both '
+            'constraints were unbounded, this render object doesn\'t know how much '
+            'size to consume.'
+          )
+        ]);
       }
       return true;
     }());
@@ -1976,16 +1978,16 @@ class RenderDecoratedBox extends RenderProxyBox {
       _painter.paint(context.canvas, offset, filledConfiguration);
       assert(() {
         if (debugSaveCount != context.canvas.getSaveCount()) {
-          throw FlutterError(
-            '${_decoration.runtimeType} painter had mismatching save and restore calls.\n'
-            'Before painting the decoration, the canvas save count was $debugSaveCount. '
-            'After painting it, the canvas save count was ${context.canvas.getSaveCount()}. '
-            'Every call to save() or saveLayer() must be matched by a call to restore().\n'
-            'The decoration was:\n'
-            '  $decoration\n'
-            'The painter was:\n'
-            '  $_painter'
-          );
+          throw FlutterError.fromParts(<DiagnosticsNode>[
+            ErrorSummary('${_decoration.runtimeType} painter had mismatching save and restore calls.'),
+            ErrorDescription(
+              'Before painting the decoration, the canvas save count was $debugSaveCount. '
+              'After painting it, the canvas save count was ${context.canvas.getSaveCount()}. '
+              'Every call to save() or saveLayer() must be matched by a call to restore().'
+            ),
+            DiagnosticsProperty<Decoration>('The decoration was', decoration, style: DiagnosticsTreeStyle.errorProperty),
+            DiagnosticsProperty<BoxPainter>('The painter was', _painter, style: DiagnosticsTreeStyle.errorProperty),
+          ]);
         }
         return true;
       }());

packages/flutter/lib/src/rendering/sliver_persistent_header.dart

@@ -132,11 +132,11 @@ abstract class RenderSliverPersistentHeader extends RenderSliver with RenderObje
     assert(() {
       if (minExtent <= maxExtent)
         return true;
-      throw FlutterError(
-        'The maxExtent for this $runtimeType is less than its minExtent.\n'
-        'The specified maxExtent was: ${maxExtent.toStringAsFixed(1)}\n'
-        'The specified minExtent was: ${minExtent.toStringAsFixed(1)}\n'
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('The maxExtent for this $runtimeType is less than its minExtent.'),
+        DoubleProperty('The specified maxExtent was', maxExtent),
+        DoubleProperty('The specified minExtent was', minExtent),
+      ]);
     }());
     child?.layout(
       constraints.asBoxConstraints(maxExtent: math.max(minExtent, maxExtent - shrinkOffset)),

packages/flutter/lib/src/rendering/viewport.dart

@@ -296,14 +296,18 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
     assert(() {
       if (!RenderObject.debugCheckingIntrinsics) {
         assert(this is! RenderShrinkWrappingViewport); // it has its own message
-        throw FlutterError(
-          '$runtimeType does not support returning intrinsic dimensions.\n'
-          'Calculating the intrinsic dimensions would require instantiating every child of '
-          'the viewport, which defeats the point of viewports being lazy.\n'
-          'If you are merely trying to shrink-wrap the viewport in the main axis direction, '
-          'consider a RenderShrinkWrappingViewport render object (ShrinkWrappingViewport widget), '
-          'which achieves that effect without implementing the intrinsic dimension API.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('$runtimeType does not support returning intrinsic dimensions.'),
+          ErrorDescription(
+            'Calculating the intrinsic dimensions would require instantiating every child of '
+            'the viewport, which defeats the point of viewports being lazy.',
+          ),
+          ErrorHint(
+            'If you are merely trying to shrink-wrap the viewport in the main axis direction, '
+            'consider a RenderShrinkWrappingViewport render object (ShrinkWrappingViewport widget), '
+            'which achieves that effect without implementing the intrinsic dimension API.'
+          ),
+        ]);
       }
       return true;
     }());
@@ -1165,19 +1169,23 @@ class RenderViewport extends RenderViewportBase<SliverPhysicalContainerParentDat
         switch (axis) {
           case Axis.vertical:
             if (!constraints.hasBoundedHeight) {
-              throw FlutterError(
-                'Vertical viewport was given unbounded height.\n'
-                'Viewports expand in the scrolling direction to fill their container. '
-                'In this case, a vertical viewport was given an unlimited amount of '
-                'vertical space in which to expand. This situation typically happens '
-                'when a scrollable widget is nested inside another scrollable widget.\n'
-                'If this widget is always nested in a scrollable widget there '
-                'is no need to use a viewport because there will always be enough '
-                'vertical space for the children. In this case, consider using a '
-                'Column instead. Otherwise, consider using the "shrinkWrap" property '
-                '(or a ShrinkWrappingViewport) to size the height of the viewport '
-                'to the sum of the heights of its children.'
-              );
+              throw FlutterError.fromParts(<DiagnosticsNode>[
+                ErrorSummary('Vertical viewport was given unbounded height.'),
+                ErrorDescription(
+                  'Viewports expand in the scrolling direction to fill their container. '
+                  'In this case, a vertical viewport was given an unlimited amount of '
+                  'vertical space in which to expand. This situation typically happens '
+                  'when a scrollable widget is nested inside another scrollable widget.'
+                ),
+                ErrorHint(
+                  'If this widget is always nested in a scrollable widget there '
+                  'is no need to use a viewport because there will always be enough '
+                  'vertical space for the children. In this case, consider using a '
+                  'Column instead. Otherwise, consider using the "shrinkWrap" property '
+                  '(or a ShrinkWrappingViewport) to size the height of the viewport '
+                  'to the sum of the heights of its children.'
+                )
+              ]);
             }
             if (!constraints.hasBoundedWidth) {
               throw FlutterError(
@@ -1191,19 +1199,23 @@ class RenderViewport extends RenderViewportBase<SliverPhysicalContainerParentDat
             break;
           case Axis.horizontal:
             if (!constraints.hasBoundedWidth) {
-              throw FlutterError(
-                'Horizontal viewport was given unbounded width.\n'
-                'Viewports expand in the scrolling direction to fill their container.'
-                'In this case, a horizontal viewport was given an unlimited amount of '
-                'horizontal space in which to expand. This situation typically happens '
-                'when a scrollable widget is nested inside another scrollable widget.\n'
-                'If this widget is always nested in a scrollable widget there '
-                'is no need to use a viewport because there will always be enough '
-                'horizontal space for the children. In this case, consider using a '
-                'Row instead. Otherwise, consider using the "shrinkWrap" property '
-                '(or a ShrinkWrappingViewport) to size the width of the viewport '
-                'to the sum of the widths of its children.'
-              );
+              throw FlutterError.fromParts(<DiagnosticsNode>[
+                ErrorSummary('Horizontal viewport was given unbounded width.'),
+                ErrorDescription(
+                  'Viewports expand in the scrolling direction to fill their container.'
+                  'In this case, a horizontal viewport was given an unlimited amount of '
+                  'horizontal space in which to expand. This situation typically happens '
+                  'when a scrollable widget is nested inside another scrollable widget.'
+                ),
+                ErrorHint(
+                  'If this widget is always nested in a scrollable widget there '
+                  'is no need to use a viewport because there will always be enough '
+                  'horizontal space for the children. In this case, consider using a '
+                  'Row instead. Otherwise, consider using the "shrinkWrap" property '
+                  '(or a ShrinkWrappingViewport) to size the width of the viewport '
+                  'to the sum of the widths of its children.'
+                )
+              ]);
             }
             if (!constraints.hasBoundedHeight) {
               throw FlutterError(
@@ -1585,14 +1597,18 @@ class RenderShrinkWrappingViewport extends RenderViewportBase<SliverLogicalConta
   bool debugThrowIfNotCheckingIntrinsics() {
     assert(() {
       if (!RenderObject.debugCheckingIntrinsics) {
-        throw FlutterError(
-          '$runtimeType does not support returning intrinsic dimensions.\n'
-          'Calculating the intrinsic dimensions would require instantiating every child of '
-          'the viewport, which defeats the point of viewports being lazy.\n'
-          'If you are merely trying to shrink-wrap the viewport in the main axis direction, '
-          'you should be able to achieve that effect by just giving the viewport loose '
-          'constraints, without needing to measure its intrinsic dimensions.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('$runtimeType does not support returning intrinsic dimensions.'),
+          ErrorDescription(
+           'Calculating the intrinsic dimensions would require instantiating every child of '
+           'the viewport, which defeats the point of viewports being lazy.'
+          ),
+          ErrorHint(
+            'If you are merely trying to shrink-wrap the viewport in the main axis direction, '
+            'you should be able to achieve that effect by just giving the viewport loose '
+            'constraints, without needing to measure its intrinsic dimensions.'
+          )
+        ]);
       }
       return true;
     }());

packages/flutter/lib/src/scheduler/binding.dart

@@ -94,15 +94,19 @@ class _FrameCallbackEntry {
       if (rescheduling) {
         assert(() {
           if (debugCurrentCallbackStack == null) {
-            throw FlutterError(
-              'scheduleFrameCallback called with rescheduling true, but no callback is in scope.\n'
-              'The "rescheduling" argument should only be set to true if the '
-              'callback is being reregistered from within the callback itself, '
-              'and only then if the callback itself is entirely synchronous. '
-              'If this is the initial registration of the callback, or if the '
-              'callback is asynchronous, then do not use the "rescheduling" '
-              'argument.'
-            );
+            throw FlutterError.fromParts(<DiagnosticsNode>[
+              ErrorSummary('scheduleFrameCallback called with rescheduling true, but no callback is in scope.'),
+              ErrorDescription(
+                'The "rescheduling" argument should only be set to true if the '
+                'callback is being reregistered from within the callback itself, '
+                'and only then if the callback itself is entirely synchronous.'
+              ),
+              ErrorHint(
+                'If this is the initial registration of the callback, or if the '
+                'callback is asynchronous, then do not use the "rescheduling" '
+                'argument.'
+              )
+            ]);
           }
           return true;
         }());

packages/flutter/lib/src/scheduler/ticker.dart

@@ -38,6 +38,8 @@ abstract class TickerProvider {
   Ticker createTicker(TickerCallback onTick);
 }
 
+// TODO(jacobr): make Ticker Diagnosticable to simplify reporting errors
+// related to a ticker.
 /// Calls its callback once per animation frame.
 ///
 /// When created, a ticker is initially disabled. Call [start] to
@@ -145,11 +147,11 @@ class Ticker {
   TickerFuture start() {
     assert(() {
       if (isActive) {
-        throw FlutterError(
-          'A ticker was started twice.\n'
-          'A ticker that is already active cannot be started again without first stopping it.\n'
-          'The affected ticker was: ${ toString(debugIncludeStack: true) }'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('A ticker was started twice.'),
+          ErrorDescription('A ticker that is already active cannot be started again without first stopping it.'),
+          describeForError('The affected ticker was'),
+        ]);
       }
       return true;
     }());
@@ -164,6 +166,13 @@ class Ticker {
     return _future;
   }
 
+  /// Adds a debug representation of a [Ticker] optimized for including in error
+  /// messages.
+  DiagnosticsNode describeForError(String name) {
+    // TODO(jacobr): make this more structured.
+    return DiagnosticsProperty<Ticker>(name, this, description: toString(debugIncludeStack: true));
+  }
+
   /// Stops calling this [Ticker]'s callback.
   ///
   /// If called with the `canceled` argument set to false (the default), causes

packages/flutter/lib/src/services/asset_bundle.dart

@@ -116,10 +116,10 @@ class NetworkAssetBundle extends AssetBundle {
     final HttpClientRequest request = await _httpClient.getUrl(_urlFromKey(key));
     final HttpClientResponse response = await request.close();
     if (response.statusCode != HttpStatus.ok)
-      throw FlutterError(
-        'Unable to load asset: $key\n'
-        'HTTP status code: ${response.statusCode}'
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('Unable to load asset: $key'),
+        IntProperty('HTTP status code', response.statusCode),
+      ]);
     final Uint8List bytes = await consolidateHttpClientResponseBytes(response);
     return bytes.buffer.asByteData();
   }

packages/flutter/lib/src/services/text_input.dart

@@ -765,7 +765,7 @@ TextInputAction _toTextInputAction(String action) {
     case 'TextInputAction.newline':
       return TextInputAction.newline;
   }
-  throw FlutterError('Unknown text input action: $action');
+  throw FlutterError.fromParts(<DiagnosticsNode>[ErrorSummary('Unknown text input action: $action')]);
 }
 
 FloatingCursorDragState _toTextCursorAction(String state) {
@@ -777,7 +777,7 @@ FloatingCursorDragState _toTextCursorAction(String state) {
     case 'FloatingCursorDragState.end':
       return FloatingCursorDragState.End;
   }
-  throw FlutterError('Unknown text cursor action: $state');
+  throw FlutterError.fromParts(<DiagnosticsNode>[ErrorSummary('Unknown text cursor action: $state')]);
 }
 
 RawFloatingCursorPoint _toTextPoint(FloatingCursorDragState state, Map<String, dynamic> encoded) {

packages/flutter/lib/src/widgets/animated_list.dart

@@ -391,16 +391,17 @@ class AnimatedList extends StatefulWidget {
     final AnimatedListState result = context.ancestorStateOfType(const TypeMatcher<AnimatedListState>());
     if (nullOk || result != null)
       return result;
-    throw FlutterError(
-      'AnimatedList.of() called with a context that does not contain an AnimatedList.\n'
-      'No AnimatedList ancestor could be found starting from the context that '
-      'was passed to AnimatedList.of(). This can happen when the context '
-      'provided is from the same StatefulWidget that built the AnimatedList. '
-      'Please see the AnimatedList documentation for examples of how to refer to '
-      'an AnimatedListState object: https://api.flutter.dev/flutter/widgets/AnimatedListState-class.html \n'
-      'The context used was:\n'
-      '  $context'
-    );
+    throw FlutterError.fromParts(<DiagnosticsNode>[
+      ErrorSummary('AnimatedList.of() called with a context that does not contain an AnimatedList.'),
+      ErrorDescription('No AnimatedList ancestor could be found starting from the context that was passed to AnimatedList.of().'),
+      ErrorHint(
+        'This can happen when the context provided is from the same StatefulWidget that '
+        'built the AnimatedList. Please see the AnimatedList documentation for examples '
+        'of how to refer to an AnimatedListState object:'
+        '  https://api.flutter.dev/flutter/widgets/AnimatedListState-class.html'
+      ),
+      context.describeElement('The context used was')
+    ]);
   }
 
   @override

packages/flutter/lib/src/widgets/app.dart

@@ -783,12 +783,14 @@ class _WidgetsAppState extends State<WidgetsApp> with WidgetsBindingObserver {
     final Route<dynamic> result = widget.onUnknownRoute(settings);
     assert(() {
       if (result == null) {
-        throw FlutterError(
-          'The onUnknownRoute callback returned null.\n'
-          'When the $runtimeType requested the route $settings from its '
-          'onUnknownRoute callback, the callback returned null. Such callbacks '
-          'must never return null.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('The onUnknownRoute callback returned null.'),
+          ErrorDescription(
+            'When the $runtimeType requested the route $settings from its '
+            'onUnknownRoute callback, the callback returned null. Such callbacks '
+            'must never return null.'
+          )
+        ]);
       }
       return true;
     }());

packages/flutter/lib/src/widgets/binding.dart

@@ -645,23 +645,27 @@ mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureB
     // should not trigger a new frame.
     assert(() {
       if (debugBuildingDirtyElements) {
-        throw FlutterError(
-          'Build scheduled during frame.\n'
-          'While the widget tree was being built, laid out, and painted, '
-          'a new frame was scheduled to rebuild the widget tree. '
-          'This might be because setState() was called from a layout or '
-          'paint callback. '
-          'If a change is needed to the widget tree, it should be applied '
-          'as the tree is being built. Scheduling a change for the subsequent '
-          'frame instead results in an interface that lags behind by one frame. '
-          'If this was done to make your build dependent on a size measured at '
-          'layout time, consider using a LayoutBuilder, CustomSingleChildLayout, '
-          'or CustomMultiChildLayout. If, on the other hand, the one frame delay '
-          'is the desired effect, for example because this is an '
-          'animation, consider scheduling the frame in a post-frame callback '
-          'using SchedulerBinding.addPostFrameCallback or '
-          'using an AnimationController to trigger the animation.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('Build scheduled during frame.'),
+          ErrorDescription(
+            'While the widget tree was being built, laid out, and painted, '
+            'a new frame was scheduled to rebuild the widget tree.'
+          ),
+          ErrorHint(
+            'This might be because setState() was called from a layout or '
+            'paint callback. '
+            'If a change is needed to the widget tree, it should be applied '
+            'as the tree is being built. Scheduling a change for the subsequent '
+            'frame instead results in an interface that lags behind by one frame. '
+            'If this was done to make your build dependent on a size measured at '
+            'layout time, consider using a LayoutBuilder, CustomSingleChildLayout, '
+            'or CustomMultiChildLayout. If, on the other hand, the one frame delay '
+            'is the desired effect, for example because this is an '
+            'animation, consider scheduling the frame in a post-frame callback '
+            'using SchedulerBinding.addPostFrameCallback or '
+            'using an AnimationController to trigger the animation.',
+          )
+        ]);
       }
       return true;
     }());

packages/flutter/lib/src/widgets/debug.dart

@@ -5,6 +5,8 @@
 import 'dart:collection';
 import 'dart:developer' show Timeline; // to disambiguate reference in dartdocs below
 
+import 'package:flutter/foundation.dart';
+
 import 'basic.dart';
 import 'framework.dart';
 import 'media_query.dart';
@@ -126,11 +128,13 @@ bool debugChildrenHaveDuplicateKeys(Widget parent, Iterable<Widget> children) {
   assert(() {
     final Key nonUniqueKey = _firstNonUniqueKey(children);
     if (nonUniqueKey != null) {
-      throw FlutterError(
-        'Duplicate keys found.\n'
-        'If multiple keyed nodes exist as children of another node, they must have unique keys.\n'
-        '$parent has multiple children with key $nonUniqueKey.'
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('Duplicate keys found.'),
+        ErrorDescription(
+          'If multiple keyed nodes exist as children of another node, they must have unique keys.\n'
+          '$parent has multiple children with key $nonUniqueKey.'
+        ),
+      ]);
     }
     return true;
   }());
@@ -153,7 +157,9 @@ bool debugItemsHaveDuplicateKeys(Iterable<Widget> items) {
   assert(() {
     final Key nonUniqueKey = _firstNonUniqueKey(items);
     if (nonUniqueKey != null)
-      throw FlutterError('Duplicate key found: $nonUniqueKey.');
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('Duplicate key found: $nonUniqueKey.'),
+      ]);
     return true;
   }());
   return false;
@@ -174,15 +180,12 @@ bool debugItemsHaveDuplicateKeys(Iterable<Widget> items) {
 bool debugCheckHasTable(BuildContext context) {
   assert(() {
     if (context.widget is! Table && context.ancestorWidgetOfExactType(Table) == null) {
-      final Element element = context;
-      throw FlutterError(
-        'No Table widget found.\n'
-        '${context.widget.runtimeType} widgets require a Table widget ancestor.\n'
-        'The specific widget that could not find a Table ancestor was:\n'
-        '  ${context.widget}\n'
-        'The ownership chain for the affected widget is:\n'
-        '  ${element.debugGetCreatorChain(10)}'
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('No Table widget found.'),
+        ErrorDescription('${context.widget.runtimeType} widgets require a Table widget ancestor.'),
+        context.describeWidget('The specific widget that could not find a Table ancestor was'),
+        context.describeOwnershipChain('The ownership chain for the affected widget is'),
+      ]);
     }
     return true;
   }());
@@ -205,17 +208,16 @@ bool debugCheckHasTable(BuildContext context) {
 bool debugCheckHasMediaQuery(BuildContext context) {
   assert(() {
     if (context.widget is! MediaQuery && context.ancestorWidgetOfExactType(MediaQuery) == null) {
-      final Element element = context;
-      throw FlutterError(
-        'No MediaQuery widget found.\n'
-        '${context.widget.runtimeType} widgets require a MediaQuery widget ancestor.\n'
-        'The specific widget that could not find a MediaQuery ancestor was:\n'
-        '  ${context.widget}\n'
-        'The ownership chain for the affected widget is:\n'
-        '  ${element.debugGetCreatorChain(10)}\n'
-        'Typically, the MediaQuery widget is introduced by the MaterialApp or '
-        'WidgetsApp widget at the top of your application widget tree.'
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('No MediaQuery widget found.'),
+        ErrorDescription('${context.widget.runtimeType} widgets require a MediaQuery widget ancestor.'),
+        context.describeWidget('The specific widget that could not find a MediaQuery ancestor was'),
+        context.describeOwnershipChain('The ownership chain for the affected widget is'),
+        ErrorHint(
+          'Typically, the MediaQuery widget is introduced by the MaterialApp or '
+          'WidgetsApp widget at the top of your application widget tree.'
+        ),
+      ]);
     }
     return true;
   }());
@@ -238,21 +240,20 @@ bool debugCheckHasMediaQuery(BuildContext context) {
 bool debugCheckHasDirectionality(BuildContext context) {
   assert(() {
     if (context.widget is! Directionality && context.ancestorWidgetOfExactType(Directionality) == null) {
-      final Element element = context;
-      throw FlutterError(
-        'No Directionality widget found.\n'
-        '${context.widget.runtimeType} widgets require a Directionality widget ancestor.\n'
-        'The specific widget that could not find a Directionality ancestor was:\n'
-        '  ${context.widget}\n'
-        'The ownership chain for the affected widget is:\n'
-        '  ${element.debugGetCreatorChain(10)}\n'
-        'Typically, the Directionality widget is introduced by the MaterialApp '
-        'or WidgetsApp widget at the top of your application widget tree. It '
-        'determines the ambient reading direction and is used, for example, to '
-        'determine how to lay out text, how to interpret "start" and "end" '
-        'values, and to resolve EdgeInsetsDirectional, '
-        'AlignmentDirectional, and other *Directional objects.'
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('No Directionality widget found.'),
+        ErrorDescription('${context.widget.runtimeType} widgets require a Directionality widget ancestor.\n'),
+        context.describeWidget('The specific widget that could not find a Directionality ancestor was'),
+        context.describeOwnershipChain('The ownership chain for the affected widget is'),
+        ErrorHint(
+          'Typically, the Directionality widget is introduced by the MaterialApp '
+          'or WidgetsApp widget at the top of your application widget tree. It '
+          'determines the ambient reading direction and is used, for example, to '
+          'determine how to lay out text, how to interpret "start" and "end" '
+          'values, and to resolve EdgeInsetsDirectional, '
+          'AlignmentDirectional, and other *Directional objects.'
+        ),
+      ]);
     }
     return true;
   }());
@@ -268,21 +269,25 @@ bool debugCheckHasDirectionality(BuildContext context) {
 void debugWidgetBuilderValue(Widget widget, Widget built) {
   assert(() {
     if (built == null) {
-      throw FlutterError(
-        'A build function returned null.\n'
-        'The offending widget is: $widget\n'
-        'Build functions must never return null. '
-        'To return an empty space that causes the building widget to fill available room, return "new Container()". '
-        'To return an empty space that takes as little room as possible, return "new Container(width: 0.0, height: 0.0)".'
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('A build function returned null.'),
+        DiagnosticsProperty<Widget>('The offending widget is', widget, style: DiagnosticsTreeStyle.errorProperty),
+        ErrorDescription('Build functions must never return null.'),
+        ErrorHint(
+          'To return an empty space that causes the building widget to fill available room, return "Container()". '
+          'To return an empty space that takes as little room as possible, return "Container(width: 0.0, height: 0.0)".'
+        ),
+      ]);
     }
     if (widget == built) {
-      throw FlutterError(
-        'A build function returned context.widget.\n'
-        'The offending widget is: $widget\n'
-        'Build functions must never return their BuildContext parameter\'s widget or a child that contains "context.widget". '
-        'Doing so introduces a loop in the widget tree that can cause the app to crash.'
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('A build function returned context.widget.'),
+        DiagnosticsProperty<Widget>('The offending widget is', widget, style: DiagnosticsTreeStyle.errorProperty),
+        ErrorDescription(
+          'Build functions must never return their BuildContext parameter\'s widget or a child that contains "context.widget". '
+          'Doing so introduces a loop in the widget tree that can cause the app to crash.'
+        ),
+      ]);
     }
     return true;
   }());
@@ -302,7 +307,7 @@ bool debugAssertAllWidgetVarsUnset(String reason) {
         debugPrintGlobalKeyedWidgetLifecycle ||
         debugProfileBuildsEnabled ||
         debugHighlightDeprecatedWidgets) {
-      throw FlutterError(reason);
+      throw FlutterError.fromParts(<DiagnosticsNode>[ErrorSummary('$reason')]);
     }
     return true;
   }());

packages/flutter/lib/src/widgets/dismissible.dart

@@ -523,11 +523,13 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
       assert(() {
         if (_resizeAnimation.status != AnimationStatus.forward) {
           assert(_resizeAnimation.status == AnimationStatus.completed);
-          throw FlutterError(
-            'A dismissed Dismissible widget is still part of the tree.\n'
-            'Make sure to implement the onDismissed handler and to immediately remove the Dismissible\n'
-            'widget from the application once that handler has fired.'
-          );
+          throw FlutterError.fromParts(<DiagnosticsNode>[
+            ErrorSummary('A dismissed Dismissible widget is still part of the tree.'),
+            ErrorHint(
+              'Make sure to implement the onDismissed handler and to immediately remove the Dismissible '
+              'widget from the application once that handler has fired.'
+            )
+          ]);
         }
         return true;
       }());

packages/flutter/lib/src/widgets/editable_text.dart

@@ -190,7 +190,7 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
   /// change the controller's [value].
   set selection(TextSelection newSelection) {
     if (newSelection.start > text.length || newSelection.end > text.length)
-      throw FlutterError('invalid text selection: $newSelection');
+      throw FlutterError.fromParts(<DiagnosticsNode>[ErrorSummary('invalid text selection: $newSelection')]);
     value = value.copyWith(selection: newSelection, composing: TextRange.empty);
   }
 

packages/flutter/lib/src/widgets/gesture_detector.dart

@@ -232,18 +232,23 @@ class GestureDetector extends StatelessWidget {
          final bool haveScale = onScaleStart != null || onScaleUpdate != null || onScaleEnd != null;
          if (havePan || haveScale) {
            if (havePan && haveScale) {
-             throw FlutterError(
-               'Incorrect GestureDetector arguments.\n'
-               'Having both a pan gesture recognizer and a scale gesture recognizer is redundant; scale is a superset of pan. Just use the scale gesture recognizer.'
-             );
+             throw FlutterError.fromParts(<DiagnosticsNode>[
+               ErrorSummary('Incorrect GestureDetector arguments.'),
+               ErrorDescription(
+                 'Having both a pan gesture recognizer and a scale gesture recognizer is redundant; scale is a superset of pan.'
+               ),
+               ErrorHint('Just use the scale gesture recognizer.')
+             ]);
            }
            final String recognizer = havePan ? 'pan' : 'scale';
            if (haveVerticalDrag && haveHorizontalDrag) {
-             throw FlutterError(
-               'Incorrect GestureDetector arguments.\n'
-               'Simultaneously having a vertical drag gesture recognizer, a horizontal drag gesture recognizer, and a $recognizer gesture recognizer '
-               'will result in the $recognizer gesture recognizer being ignored, since the other two will catch all drags.'
-             );
+             throw FlutterError.fromParts(<DiagnosticsNode>[
+               ErrorSummary('Incorrect GestureDetector arguments.'),
+               ErrorDescription(
+                 'Simultaneously having a vertical drag gesture recognizer, a horizontal drag gesture recognizer, and a $recognizer gesture recognizer '
+                 'will result in the $recognizer gesture recognizer being ignored, since the other two will catch all drags.'
+               )
+             ]);
            }
          }
          return true;
@@ -935,13 +940,15 @@ class RawGestureDetectorState extends State<RawGestureDetector> {
   void replaceGestureRecognizers(Map<Type, GestureRecognizerFactory> gestures) {
     assert(() {
       if (!context.findRenderObject().owner.debugDoingLayout) {
-        throw FlutterError(
-          'Unexpected call to replaceGestureRecognizers() method of RawGestureDetectorState.\n'
-          'The replaceGestureRecognizers() method can only be called during the layout phase. '
-          'To set the gesture recognizers at other times, trigger a new build using setState() '
-          'and provide the new gesture recognizers as constructor arguments to the corresponding '
-          'RawGestureDetector or GestureDetector object.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('Unexpected call to replaceGestureRecognizers() method of RawGestureDetectorState.'),
+          ErrorDescription('The replaceGestureRecognizers() method can only be called during the layout phase.'),
+          ErrorHint(
+            'To set the gesture recognizers at other times, trigger a new build using setState() '
+            'and provide the new gesture recognizers as constructor arguments to the corresponding '
+            'RawGestureDetector or GestureDetector object.'
+          )
+        ]);
       }
       return true;
     }());

packages/flutter/lib/src/widgets/heroes.dart

@@ -262,14 +262,15 @@ class Hero extends StatefulWidget {
     void inviteHero(StatefulElement hero, Object tag) {
       assert(() {
         if (result.containsKey(tag)) {
-          throw FlutterError(
-            'There are multiple heroes that share the same tag within a subtree.\n'
-            'Within each subtree for which heroes are to be animated (i.e. a PageRoute subtree), '
-            'each Hero must have a unique non-null tag.\n'
-            'In this case, multiple heroes had the following tag: $tag\n'
-            'Here is the subtree for one of the offending heroes:\n'
-            '${hero.toStringDeep(prefixLineOne: "# ")}'
-          );
+          throw FlutterError.fromParts(<DiagnosticsNode>[
+            ErrorSummary('There are multiple heroes that share the same tag within a subtree.'),
+            ErrorDescription(
+              'Within each subtree for which heroes are to be animated (i.e. a PageRoute subtree), '
+              'each Hero must have a unique non-null tag.\n'
+              'In this case, multiple heroes had the following tag: $tag\n'
+            ),
+            DiagnosticsProperty<StatefulElement>('Here is the subtree for one of the offending heroes', hero, linePrefix: '# ', style: DiagnosticsTreeStyle.dense),
+          ]);
         }
         return true;
       }());

packages/flutter/lib/src/widgets/media_query.dart

@@ -795,15 +795,16 @@ class MediaQuery extends InheritedWidget {
       return query.data;
     if (nullOk)
       return null;
-    throw FlutterError(
-      'MediaQuery.of() called with a context that does not contain a MediaQuery.\n'
-      'No MediaQuery ancestor could be found starting from the context that was passed '
-      'to MediaQuery.of(). This can happen because you do not have a WidgetsApp or '
-      'MaterialApp widget (those widgets introduce a MediaQuery), or it can happen '
-      'if the context you use comes from a widget above those widgets.\n'
-      'The context used was:\n'
-      '  $context'
-    );
+    throw FlutterError.fromParts(<DiagnosticsNode>[
+      ErrorSummary('MediaQuery.of() called with a context that does not contain a MediaQuery.'),
+      ErrorDescription(
+        'No MediaQuery ancestor could be found starting from the context that was passed '
+        'to MediaQuery.of(). This can happen because you do not have a WidgetsApp or '
+        'MaterialApp widget (those widgets introduce a MediaQuery), or it can happen '
+        'if the context you use comes from a widget above those widgets.'
+      ),
+      context.describeElement('The context used was')
+    ]);
   }
 
   /// Returns textScaleFactor for the nearest MediaQuery ancestor or 1.0, if

packages/flutter/lib/src/widgets/navigator.dart

@@ -1630,26 +1630,28 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin {
     if (route == null && !allowNull) {
       assert(() {
         if (widget.onUnknownRoute == null) {
-          throw FlutterError(
-            'If a Navigator has no onUnknownRoute, then its onGenerateRoute must never return null.\n'
-            'When trying to build the route "$name", onGenerateRoute returned null, but there was no '
-            'onUnknownRoute callback specified.\n'
-            'The Navigator was:\n'
-            '  $this'
-          );
+          throw FlutterError.fromParts(<DiagnosticsNode>[
+            ErrorSummary('If a Navigator has no onUnknownRoute, then its onGenerateRoute must never return null.'),
+            ErrorDescription(
+              'When trying to build the route "$name", onGenerateRoute returned null, but there was no '
+              'onUnknownRoute callback specified.'
+            ),
+            DiagnosticsProperty<NavigatorState>('The Navigator was', this, style: DiagnosticsTreeStyle.errorProperty),
+          ]);
         }
         return true;
       }());
       route = widget.onUnknownRoute(settings);
       assert(() {
         if (route == null) {
-          throw FlutterError(
-            'A Navigator\'s onUnknownRoute returned null.\n'
-            'When trying to build the route "$name", both onGenerateRoute and onUnknownRoute returned '
-            'null. The onUnknownRoute callback should never return null.\n'
-            'The Navigator was:\n'
-            '  $this'
-          );
+          throw FlutterError.fromParts(<DiagnosticsNode>[
+            ErrorSummary('A Navigator\'s onUnknownRoute returned null.'),
+            ErrorDescription(
+              'When trying to build the route "$name", both onGenerateRoute and onUnknownRoute returned '
+              'null. The onUnknownRoute callback should never return null.'
+            ),
+            DiagnosticsProperty<NavigatorState>('The Navigator was', this, style: DiagnosticsTreeStyle.errorProperty),
+          ]);
         }
         return true;
       }());

packages/flutter/lib/src/widgets/overlay.dart

@@ -238,17 +238,16 @@ class Overlay extends StatefulWidget {
     final OverlayState result = context.ancestorStateOfType(const TypeMatcher<OverlayState>());
     assert(() {
       if (debugRequiredFor != null && result == null) {
-        final String additional = context.widget != debugRequiredFor
-          ? '\nThe context from which that widget was searching for an overlay was:\n  $context'
-          : '';
-        throw FlutterError(
-          'No Overlay widget found.\n'
-          '${debugRequiredFor.runtimeType} widgets require an Overlay widget ancestor for correct operation.\n'
-          'The most common way to add an Overlay to an application is to include a MaterialApp or Navigator widget in the runApp() call.\n'
-          'The specific widget that failed to find an overlay was:\n'
-          '  $debugRequiredFor'
-          '$additional'
-        );
+        final List<DiagnosticsNode> information = <DiagnosticsNode>[
+          ErrorSummary('No Overlay widget found.'),
+          ErrorDescription('${debugRequiredFor.runtimeType} widgets require an Overlay widget ancestor for correct operation.'),
+          ErrorHint('The most common way to add an Overlay to an application is to include a MaterialApp or Navigator widget in the runApp() call.'),
+          DiagnosticsProperty<Widget>('The specific widget that failed to find an overlay was', debugRequiredFor, style: DiagnosticsTreeStyle.errorProperty),
+          if (context.widget != debugRequiredFor)
+            context.describeElement('The context from which that widget was searching for an overlay was')
+        ];
+
+        throw FlutterError.fromParts(information);
       }
       return true;
     }());

packages/flutter/lib/src/widgets/scroll_physics.dart

@@ -422,17 +422,17 @@ class ClampingScrollPhysics extends ScrollPhysics {
   double applyBoundaryConditions(ScrollMetrics position, double value) {
     assert(() {
       if (value == position.pixels) {
-        throw FlutterError(
-          '$runtimeType.applyBoundaryConditions() was called redundantly.\n'
-          'The proposed new position, $value, is exactly equal to the current position of the '
-          'given ${position.runtimeType}, ${position.pixels}.\n'
-          'The applyBoundaryConditions method should only be called when the value is '
-          'going to actually change the pixels, otherwise it is redundant.\n'
-          'The physics object in question was:\n'
-          '  $this\n'
-          'The position object in question was:\n'
-          '  $position'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('$runtimeType.applyBoundaryConditions() was called redundantly.'),
+          ErrorDescription(
+            'The proposed new position, $value, is exactly equal to the current position of the '
+            'given ${position.runtimeType}, ${position.pixels}.\n'
+            'The applyBoundaryConditions method should only be called when the value is '
+            'going to actually change the pixels, otherwise it is redundant.'
+          ),
+          DiagnosticsProperty<ScrollPhysics>('The physics object in question was', this, style: DiagnosticsTreeStyle.errorProperty),
+          DiagnosticsProperty<ScrollMetrics>('The position object in question was', position, style: DiagnosticsTreeStyle.errorProperty)
+        ]);
       }
       return true;
     }());

packages/flutter/lib/src/widgets/scroll_position.dart

@@ -203,12 +203,14 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
       assert(() {
         final double delta = newPixels - pixels;
         if (overscroll.abs() > delta.abs()) {
-          throw FlutterError(
-            '$runtimeType.applyBoundaryConditions returned invalid overscroll value.\n'
-            'setPixels() was called to change the scroll offset from $pixels to $newPixels.\n'
-            'That is a delta of $delta units.\n'
-            '$runtimeType.applyBoundaryConditions reported an overscroll of $overscroll units.'
-          );
+          throw FlutterError.fromParts(<DiagnosticsNode>[
+            ErrorSummary('$runtimeType.applyBoundaryConditions returned invalid overscroll value.'),
+            ErrorDescription(
+              'setPixels() was called to change the scroll offset from $pixels to $newPixels.\n'
+              'That is a delta of $delta units.\n'
+              '$runtimeType.applyBoundaryConditions reported an overscroll of $overscroll units.'
+            )
+          ]);
         }
         return true;
       }());
@@ -374,16 +376,18 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
     assert(() {
       final double delta = value - pixels;
       if (result.abs() > delta.abs()) {
-        throw FlutterError(
-          '${physics.runtimeType}.applyBoundaryConditions returned invalid overscroll value.\n'
-          'The method was called to consider a change from $pixels to $value, which is a '
-          'delta of ${delta.toStringAsFixed(1)} units. However, it returned an overscroll of '
-          '${result.toStringAsFixed(1)} units, which has a greater magnitude than the delta. '
-          'The applyBoundaryConditions method is only supposed to reduce the possible range '
-          'of movement, not increase it.\n'
-          'The scroll extents are $minScrollExtent .. $maxScrollExtent, and the '
-          'viewport dimension is $viewportDimension.'
-        );
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('${physics.runtimeType}.applyBoundaryConditions returned invalid overscroll value.'),
+          ErrorDescription(
+            'The method was called to consider a change from $pixels to $value, which is a '
+            'delta of ${delta.toStringAsFixed(1)} units. However, it returned an overscroll of '
+            '${result.toStringAsFixed(1)} units, which has a greater magnitude than the delta. '
+            'The applyBoundaryConditions method is only supposed to reduce the possible range '
+            'of movement, not increase it.\n'
+            'The scroll extents are $minScrollExtent .. $maxScrollExtent, and the '
+            'viewport dimension is $viewportDimension.'
+          )
+        ]);
       }
       return true;
     }());

packages/flutter/lib/src/widgets/ticker_provider.dart

@@ -82,13 +82,15 @@ mixin SingleTickerProviderStateMixin<T extends StatefulWidget> on State<T> imple
     assert(() {
       if (_ticker == null)
         return true;
-      throw FlutterError(
-        '$runtimeType is a SingleTickerProviderStateMixin but multiple tickers were created.\n'
-        'A SingleTickerProviderStateMixin can only be used as a TickerProvider once. If a '
-        'State is used for multiple AnimationController objects, or if it is passed to other '
-        'objects and those objects might use it more than one time in total, then instead of '
-        'mixing in a SingleTickerProviderStateMixin, use a regular TickerProviderStateMixin.'
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('$runtimeType is a SingleTickerProviderStateMixin but multiple tickers were created.'),
+        ErrorDescription('A SingleTickerProviderStateMixin can only be used as a TickerProvider once.'),
+        ErrorHint(
+          'If a State is used for multiple AnimationController objects, or if it is passed to other '
+          'objects and those objects might use it more than one time in total, then instead of '
+          'mixing in a SingleTickerProviderStateMixin, use a regular TickerProviderStateMixin.'
+        )
+      ]);
     }());
     _ticker = Ticker(onTick, debugLabel: kDebugMode ? 'created by $this' : null);
     // We assume that this is called from initState, build, or some sort of
@@ -103,15 +105,20 @@ mixin SingleTickerProviderStateMixin<T extends StatefulWidget> on State<T> imple
     assert(() {
       if (_ticker == null || !_ticker.isActive)
         return true;
-      throw FlutterError(
-        '$this was disposed with an active Ticker.\n'
-        '$runtimeType created a Ticker via its SingleTickerProviderStateMixin, but at the time '
-        'dispose() was called on the mixin, that Ticker was still active. The Ticker must '
-        'be disposed before calling super.dispose(). Tickers used by AnimationControllers '
-        'should be disposed by calling dispose() on the AnimationController itself. '
-        'Otherwise, the ticker will leak.\n'
-        'The offending ticker was: ${_ticker.toString(debugIncludeStack: true)}'
-      );
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('$this was disposed with an active Ticker.'),
+        ErrorDescription(
+          '$runtimeType created a Ticker via its SingleTickerProviderStateMixin, but at the time '
+          'dispose() was called on the mixin, that Ticker was still active. The Ticker must '
+          'be disposed before calling super.dispose().'
+        ),
+        ErrorHint(
+          'Tickers used by AnimationControllers '
+          'should be disposed by calling dispose() on the AnimationController itself. '
+          'Otherwise, the ticker will leak.'
+        ),
+        _ticker.describeForError('The offending ticker was')
+      ]);
     }());
     super.dispose();
   }
@@ -175,15 +182,20 @@ mixin TickerProviderStateMixin<T extends StatefulWidget> on State<T> implements
       if (_tickers != null) {
         for (Ticker ticker in _tickers) {
           if (ticker.isActive) {
-            throw FlutterError(
-              '$this was disposed with an active Ticker.\n'
-              '$runtimeType created a Ticker via its TickerProviderStateMixin, but at the time '
-              'dispose() was called on the mixin, that Ticker was still active. All Tickers must '
-              'be disposed before calling super.dispose(). Tickers used by AnimationControllers '
-              'should be disposed by calling dispose() on the AnimationController itself. '
-              'Otherwise, the ticker will leak.\n'
-              'The offending ticker was: ${ticker.toString(debugIncludeStack: true)}'
-            );
+            throw FlutterError.fromParts(<DiagnosticsNode>[
+              ErrorSummary('$this was disposed with an active Ticker.'),
+              ErrorDescription(
+                '$runtimeType created a Ticker via its TickerProviderStateMixin, but at the time '
+                'dispose() was called on the mixin, that Ticker was still active. All Tickers must '
+                'be disposed before calling super.dispose().'
+              ),
+              ErrorHint(
+                'Tickers used by AnimationControllers '
+                'should be disposed by calling dispose() on the AnimationController itself. '
+                'Otherwise, the ticker will leak.'
+              ),
+              ticker.describeForError('The offending ticker was'),
+            ]);
           }
         }
       }

packages/flutter/test/animation/animations_test.dart

@@ -238,8 +238,26 @@ void main() {
       vsync: const TestVSync(),
     );
     final CurvedAnimation curved = CurvedAnimation(parent: controller, curve: BogusCurve());
-
-    expect(() { curved.value; }, throwsFlutterError);
+    FlutterError error;
+    try {
+      curved.value;
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error.toStringDeep(), matches(
+      // RegExp matcher is required here due to flutter web and flutter mobile generating
+      // slightly different floating point numbers
+      // in Flutter web 0.0 sometimes just appears as 0. or 0
+      RegExp(
+        r'''FlutterError
+   Invalid curve endpoint at \d+(\.\d*)?\.
+   Curves must map 0\.0 to near zero and 1\.0 to near one but
+   BogusCurve mapped \d+(\.\d*)? to \d+(\.\d*)?, which is near \d+(\.\d*)?\.
+''',
+        multiLine: true
+      ),
+    ));
   });
 
   test('CurvedAnimation running with different forward and reverse durations.', () {

packages/flutter/test/cupertino/tab_test.dart

@@ -167,4 +167,68 @@ void main() {
     expect(find.text('go to second page'), findsOneWidget);
     expect(find.text('second route'), findsNothing);
   });
+
+  testWidgets('Throws FlutterError when onUnknownRoute is null', (
+      WidgetTester tester) async {
+    final GlobalKey<NavigatorState> key = GlobalKey();
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoTabView(
+          navigatorKey: key,
+          builder: (BuildContext context) => const Text('first route'),
+          onUnknownRoute: null,
+        ),
+      ),
+    );
+    FlutterError error;
+    try {
+      key.currentState.pushNamed('/2');
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   Could not find a generator for route RouteSettings("/2", null) in\n'
+      '   the _CupertinoTabViewState.\n'
+      '   Generators for routes are searched for in the following order:\n'
+      '    1. For the "/" route, the "builder" property, if non-null, is\n'
+      '   used.\n'
+      '    2. Otherwise, the "routes" table is used, if it has an entry for\n'
+      '   the route.\n'
+      '    3. Otherwise, onGenerateRoute is called. It should return a\n'
+      '   non-null value for any valid route not handled by "builder" and\n'
+      '   "routes".\n'
+      '    4. Finally if all else fails onUnknownRoute is called.\n'
+      '   Unfortunately, onUnknownRoute was not set.\n'
+    ));
+  });
+
+  testWidgets('Throws FlutterError when onUnknownRoute returns null', (
+      WidgetTester tester) async {
+    final GlobalKey<NavigatorState> key = GlobalKey<NavigatorState>();
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoTabView(
+          navigatorKey: key,
+          builder: (BuildContext context) => const Text('first route'),
+          onUnknownRoute: (_) => null,
+        ),
+      ),
+    );
+    FlutterError error;
+    try {
+      key.currentState.pushNamed('/2');
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   The onUnknownRoute callback returned null.\n'
+      '   When the _CupertinoTabViewState requested the route\n'
+      '   RouteSettings("/2", null) from its onUnknownRoute callback, the\n'
+      '   callback returned null. Such callbacks must never return null.\n'
+    ));
+  });
 }

packages/flutter/test/foundation/change_notifier_test.dart

@@ -315,4 +315,24 @@ void main() {
     expect(b.result, isTrue);
     expect(notifications, 1);
   });
+
+  test('Throws FlutterError when disposed and called', () {
+    final TestNotifier testNotifier = TestNotifier();
+    testNotifier.dispose();
+    FlutterError error;
+    try {
+      testNotifier.dispose();
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error, isFlutterError);
+    expect(error.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   A TestNotifier was used after being disposed.\n'
+      '   Once you have called dispose() on a TestNotifier, it can no\n'
+      '   longer be used.\n'
+    ));
+  });
+
 }

packages/flutter/test/material/debug_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -10,7 +11,170 @@ void main() {
     await tester.pumpWidget(const ListTile());
     final dynamic exception = tester.takeException();
     expect(exception, isFlutterError);
-    expect(exception.toString(), startsWith('No Material widget found.'));
-    expect(exception.toString(), endsWith(':\n  ListTile\nThe ancestors of this widget were:\n  [root]'));
+    final FlutterError error = exception;
+    expect(error.diagnostics.length, 5);
+    expect(error.diagnostics[2].level, DiagnosticLevel.hint);
+    expect(
+      error.diagnostics[2].toStringDeep(),
+      equalsIgnoringHashCodes(
+        'To introduce a Material widget, you can either directly include\n'
+        'one, or use a widget that contains Material itself, such as a\n'
+        'Card, Dialog, Drawer, or Scaffold.\n',
+      ),
+    );
+    expect(error.diagnostics[3], isInstanceOf<DiagnosticsProperty<Element>>());
+    expect(error.diagnostics[4], isInstanceOf<DiagnosticsBlock>());
+    expect(error.toStringDeep(),
+      'FlutterError\n'
+      '   No Material widget found.\n'
+      '   ListTile widgets require a Material widget ancestor.\n'
+      '   In material design, most widgets are conceptually "printed" on a\n'
+      '   sheet of material. In Flutter\'s material library, that material\n'
+      '   is represented by the Material widget. It is the Material widget\n'
+      '   that renders ink splashes, for instance. Because of this, many\n'
+      '   material library widgets require that there be a Material widget\n'
+      '   in the tree above them.\n'
+      '   To introduce a Material widget, you can either directly include\n'
+      '   one, or use a widget that contains Material itself, such as a\n'
+      '   Card, Dialog, Drawer, or Scaffold.\n'
+      '   The specific widget that could not find a Material ancestor was:\n'
+      '     ListTile\n'
+      '   The ancestors of this widget were:\n'
+      '     [root]\n'
+    );
+  });
+
+  testWidgets('debugCheckHasMaterialLocalizations control test', (
+      WidgetTester tester) async {
+    await tester.pumpWidget(const BackButton());
+    final dynamic exception = tester.takeException();
+    expect(exception, isFlutterError);
+    final FlutterError error = exception;
+    expect(error.diagnostics.length, 6);
+    expect(error.diagnostics[3].level, DiagnosticLevel.hint);
+    expect(
+      error.diagnostics[3].toStringDeep(),
+      equalsIgnoringHashCodes(
+        'To introduce a MaterialLocalizations, either use a MaterialApp at\n'
+        'the root of your application to include them automatically, or\n'
+        'add a Localization widget with a MaterialLocalizations delegate.\n',
+      ),
+    );
+    expect(error.diagnostics[4], isInstanceOf<DiagnosticsProperty<Element>>());
+    expect(error.diagnostics[5], isInstanceOf<DiagnosticsBlock>());
+    expect(error.toStringDeep(),
+      'FlutterError\n'
+      '   No MaterialLocalizations found.\n'
+      '   BackButton widgets require MaterialLocalizations to be provided\n'
+      '   by a Localizations widget ancestor.\n'
+      '   Localizations are used to generate many different messages,\n'
+      '   labels,and abbreviations which are used by the material library.\n'
+      '   To introduce a MaterialLocalizations, either use a MaterialApp at\n'
+      '   the root of your application to include them automatically, or\n'
+      '   add a Localization widget with a MaterialLocalizations delegate.\n'
+      '   The specific widget that could not find a MaterialLocalizations\n'
+      '   ancestor was:\n'
+      '     BackButton\n'
+      '   The ancestors of this widget were:\n'
+      '     [root]\n'
+    );
+  });
+
+  testWidgets(
+      'debugCheckHasScaffold control test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Builder(
+          builder: (BuildContext context) {
+            showBottomSheet<void>(context: context,
+                builder: (BuildContext context) => Container());
+            return Container();
+          }
+        ),
+      ),
+    );
+    final dynamic exception = tester.takeException();
+    expect(exception, isFlutterError);
+    final FlutterError error = exception;
+    expect(error.diagnostics.length, 5);
+    expect(error.diagnostics[2], isInstanceOf<DiagnosticsProperty<Element>>());
+    expect(error.diagnostics[3], isInstanceOf<DiagnosticsBlock>());
+    expect(error.diagnostics[4].level, DiagnosticLevel.hint);
+    expect(
+      error.diagnostics[4].toStringDeep(),
+      equalsIgnoringHashCodes(
+        'Typically, the Scaffold widget is introduced by the MaterialApp\n'
+        'or WidgetsApp widget at the top of your application widget tree.\n',
+      ),
+    );
+    expect(error.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   No Scaffold widget found.\n'
+      '   Builder widgets require a Scaffold widget ancestor.\n'
+      '   The specific widget that could not find a Scaffold ancestor was:\n'
+      '     Builder\n'
+      '   The ancestors of this widget were:\n'
+      '     Semantics\n'
+      '     Builder\n'
+      '     RepaintBoundary-[GlobalKey#2d465]\n'
+      '     IgnorePointer\n'
+      '     AnimatedBuilder\n'
+      '     FadeTransition\n'
+      '     FractionalTranslation\n'
+      '     SlideTransition\n'
+      '     _FadeUpwardsPageTransition\n'
+      '     AnimatedBuilder\n'
+      '     RepaintBoundary\n'
+      '     _FocusMarker\n'
+      '     Semantics\n'
+      '     FocusScope\n'
+      '     PageStorage\n'
+      '     Offstage\n'
+      '     _ModalScopeStatus\n'
+      '     _ModalScope<dynamic>-[LabeledGlobalKey<_ModalScopeState<dynamic>>#969b7]\n'
+      '     _OverlayEntry-[LabeledGlobalKey<_OverlayEntryState>#7a3ae]\n'
+      '     Stack\n'
+      '     _Theatre\n'
+      '     Overlay-[LabeledGlobalKey<OverlayState>#31a52]\n'
+      '     _FocusMarker\n'
+      '     Semantics\n'
+      '     FocusScope\n'
+      '     AbsorbPointer\n'
+      '     _PointerListener\n'
+      '     Listener\n'
+      '     Navigator-[GlobalObjectKey<NavigatorState> _WidgetsAppState#10579]\n'
+      '     IconTheme\n'
+      '     IconTheme\n'
+      '     _InheritedCupertinoTheme\n'
+      '     CupertinoTheme\n'
+      '     _InheritedTheme\n'
+      '     Theme\n'
+      '     AnimatedTheme\n'
+      '     Builder\n'
+      '     DefaultTextStyle\n'
+      '     CustomPaint\n'
+      '     Banner\n'
+      '     CheckedModeBanner\n'
+      '     Title\n'
+      '     Directionality\n'
+      '     _LocalizationsScope-[GlobalKey#a51e3]\n'
+      '     Semantics\n'
+      '     Localizations\n'
+      '     MediaQuery\n'
+      '     _MediaQueryFromWindow\n'
+      '     DefaultFocusTraversal\n'
+      '     Actions\n'
+      '     _ShortcutsMarker\n'
+      '     Semantics\n'
+      '     _FocusMarker\n'
+      '     Focus\n'
+      '     Shortcuts\n'
+      '     WidgetsApp-[GlobalObjectKey _MaterialAppState#38e79]\n'
+      '     ScrollConfiguration\n'
+      '     MaterialApp\n'
+      '     [root]\n'
+      '   Typically, the Scaffold widget is introduced by the MaterialApp\n'
+      '   or WidgetsApp widget at the top of your application widget tree.\n',
+    ));
   });
 }

packages/flutter/test/material/page_test.dart

@@ -468,7 +468,13 @@ void main() {
         ));
     await tester.pumpAndSettle();
     // An exception should've been thrown because the `builder` returned null.
-    expect(tester.takeException(), isInstanceOf<FlutterError>());
+    final dynamic exception = tester.takeException();
+    expect(exception, isInstanceOf<FlutterError>());
+    expect(exception.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   The builder for route "broken" returned null.\n'
+      '   Route builders must never return null.\n'
+    ));
   });
 
   testWidgets('test iOS edge swipe then drop back at starting point works', (WidgetTester tester) async {

packages/flutter/test/material/stepper_test.dart

@@ -529,14 +529,15 @@ void main() {
     // which will change depending on where the test is run.
     expect(lines.length, greaterThan(7));
     expect(
-      lines.take(7).join('\n'),
+      lines.take(8).join('\n'),
       equalsIgnoringHashCodes(
         '══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞════════════════════════\n'
         'The following assertion was thrown building Stepper(dirty,\n'
         'dependencies: [_LocalizationsScope-[GlobalKey#00000]], state:\n'
         '_StepperState#00000):\n'
-        'Steppers must not be nested. The material specification advises\n'
-        'that one should avoid embedding steppers within steppers.\n'
+        'Steppers must not be nested.\n'
+        'The material specification advises that one should avoid\n'
+        'embedding steppers within steppers.\n'
         'https://material.io/archive/guidelines/components/steppers.html#steppers-usage'
       ),
     );

packages/flutter/test/material/text_field_test.dart

@@ -3259,9 +3259,24 @@ void main() {
     expect(controller.selection.start, lessThanOrEqualTo(0));
     expect(controller.selection.end, lessThanOrEqualTo(0));
 
-    expect(() {
+    FlutterError error;
+    try {
       controller.selection = const TextSelection.collapsed(offset: 10);
-    }, throwsFlutterError);
+    } on FlutterError catch (e) {
+      error = e;
+    } finally {
+      expect(error, isNotNull);
+      expect(error.diagnostics.length, 1);
+      expect(
+        error.toStringDeep(),
+        equalsIgnoringHashCodes(
+          'FlutterError\n'
+          '   invalid text selection: TextSelection(baseOffset: 10,\n'
+          '   extentOffset: 10, affinity: TextAffinity.downstream,\n'
+          '   isDirectional: false)\n',
+        ),
+      );
+    }
   });
 
   testWidgets('maxLength limits input.', (WidgetTester tester) async {

packages/flutter/test/painting/border_rtl_test.dart

@@ -2,10 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart' show DiagnosticLevel, FlutterError;
 import 'package:flutter/painting.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import '../rendering/mock_canvas.dart';
+import '../rendering/rendering_tester.dart';
 
 class SillyBorder extends BoxBorder {
   @override
@@ -115,6 +117,39 @@ void main() {
     expect(() => BoxBorder.lerp(SillyBorder(), const Border(), 2.0), throwsFlutterError);
   });
 
+  test('BoxBorder.lerp throws correct FlutterError message', () {
+    FlutterError error;
+    try {
+      BoxBorder.lerp(SillyBorder(), const Border(), 2.0);
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error.diagnostics.length, 3);
+    expect(error.diagnostics[2].level, DiagnosticLevel.hint);
+    expect(
+      error.diagnostics[2].toStringDeep(),
+      equalsIgnoringHashCodes(
+        'For a more general interpolation method, consider using\n'
+        'ShapeBorder.lerp instead.\n',
+      ),
+    );
+    expect(error.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   BoxBorder.lerp can only interpolate Border and BorderDirectional\n'
+      '   classes.\n'
+      '   BoxBorder.lerp() was called with two objects of type SillyBorder\n'
+      '   and Border:\n'
+      '     SillyBorder()\n'
+      '     Border.all(BorderSide(Color(0xff000000), 0.0,\n'
+      '   BorderStyle.none))\n'
+      '   However, only Border and BorderDirectional classes are supported\n'
+      '   by this method.\n'
+      '   For a more general interpolation method, consider using\n'
+      '   ShapeBorder.lerp instead.\n'
+    ));
+  });
+
   test('BoxBorder.getInnerPath / BoxBorder.getOuterPath', () {
     // for Border, BorderDirectional
     const Border border = Border(top: BorderSide(width: 10.0), right: BorderSide(width: 20.0));

packages/flutter/test/painting/decoration_test.dart

@@ -10,8 +10,8 @@ import 'dart:ui' as ui show Image, ImageByteFormat, ColorFilter;
 import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart';
 import 'package:quiver/testing/async.dart';
-import '../flutter_test_alternative.dart';
 
+import '../flutter_test_alternative.dart';
 import '../painting/mocks_for_image_cache.dart';
 import '../rendering/rendering_tester.dart';
 
@@ -232,6 +232,43 @@ void main() {
     expect(call.positionalArguments[3].filterQuality, FilterQuality.low);
   });
 
+  test(
+      'DecorationImage with null textDirection configuration should throw Error', () {
+    final DecorationImage backgroundImage = DecorationImage(
+      image: SynchronousTestImageProvider(),
+      matchTextDirection: true,
+    );
+    final BoxDecoration boxDecoration = BoxDecoration(
+        image: backgroundImage);
+    final BoxPainter boxPainter = boxDecoration.createBoxPainter(() {
+      assert(false);
+    });
+    final TestCanvas canvas = TestCanvas(<Invocation>[]);
+    FlutterError error;
+    try {
+      boxPainter.paint(canvas, Offset.zero, const ImageConfiguration(
+          size: Size(100.0, 100.0), textDirection: null));
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error.diagnostics.length, 4);
+    expect(error.diagnostics[2], isInstanceOf<DiagnosticsProperty<DecorationImage>>());
+    expect(error.diagnostics[3], isInstanceOf<DiagnosticsProperty<ImageConfiguration>>());
+    expect(error.toStringDeep(),
+      'FlutterError\n'
+      '   ImageDecoration.matchTextDirection can only be used when a\n'
+      '   TextDirection is available.\n'
+      '   When DecorationImagePainter.paint() was called, there was no text\n'
+      '   direction provided in the ImageConfiguration object to match.\n'
+      '   The DecorationImage was:\n'
+      '     DecorationImage(SynchronousTestImageProvider(), center, match\n'
+      '     text direction)\n'
+      '   The ImageConfiguration was:\n'
+      '     ImageConfiguration(size: Size(100.0, 100.0))\n'
+    );
+  });
+
   test('BoxDecoration.lerp - shapes', () {
     // We don't lerp the shape, we just switch from one to the other at t=0.5.
     // (Use a ShapeDecoration and ShapeBorder if you want to lerp the shapes...)

packages/flutter/test/painting/text_span_test.dart

@@ -235,4 +235,29 @@ void main() {
     expect(textSpan.getSpanForPosition(const TextPosition(offset: 2)).runtimeType, WidgetSpan);
     expect(textSpan.getSpanForPosition(const TextPosition(offset: 3)).runtimeType, TextSpan);
   });
+
+  test('TextSpan with a null child should throw FlutterError', () {
+    const TextSpan text = TextSpan(
+      text: 'foo bar',
+      children: <InlineSpan>[
+        null,
+      ],
+    );
+    FlutterError error;
+    try {
+      text.computeToPlainText(StringBuffer());
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error.toStringDeep(),
+      'FlutterError\n'
+      '   TextSpan contains a null child.\n'
+      '   A TextSpan object with a non-null child list should not have any\n'
+      '   nulls in its child list.\n'
+      '   The full text in question was:\n'
+      '     TextSpan("foo bar")\n'
+    );
+  });
+
 }

packages/flutter/test/rendering/aspect_ratio_test.dart

@@ -108,14 +108,20 @@ void main() {
       ),
     );
 
-    final List<String> errorMessages = <String>[];
+    final List<FlutterErrorDetails> errors = <FlutterErrorDetails>[];
     layout(box, onErrors: () {
-      errorMessages.addAll(
-        renderer.takeAllFlutterErrorDetails().map((FlutterErrorDetails details) => '${details.exceptionAsString()}'),
-      );
+      errors.addAll(renderer.takeAllFlutterErrorDetails());
     });
-    expect(errorMessages, hasLength(2));
-    expect(errorMessages[0], contains('RenderAspectRatio has unbounded constraints.'));
+    expect(errors, hasLength(2));
+    expect(errors.first.exception, isA<FlutterError>());
+    expect(errors.first.exception.toStringDeep(),
+      'FlutterError\n'
+      '   RenderAspectRatio has unbounded constraints.\n'
+      '   This RenderAspectRatio was given an aspect ratio of 0.5 but was\n'
+      '   given both unbounded width and unbounded height constraints.\n'
+      '   Because both constraints were unbounded, this render object\n'
+      '   doesn\'t know how much size to consume.\n'
+    );
     // The second error message is a generic message generated by the Dart VM. Not worth testing.
   });
 

packages/flutter/test/rendering/debug_test.dart

@@ -151,7 +151,7 @@ void main() {
           ContainerLayer(), const Rect.fromLTRB(0.0, 0.0, 800.0, 600.0)),
         const Offset(0.0, 500),
       );
-    } catch(e) {
+    } catch (e) {
       error = e;
     }
     expect(error, isNull);
@@ -184,7 +184,7 @@ void main() {
           ContainerLayer(), const Rect.fromLTRB(0.0, 0.0, 800.0, 600.0)),
         const Offset(0.0, 500),
       );
-    } catch(e) {
+    } catch (e) {
       error = e;
     }
     expect(error, isNull);

packages/flutter/test/rendering/proxy_box_test.dart

@@ -531,6 +531,11 @@ class _FakeTicker implements Ticker {
 
   @override
   String toString({ bool debugIncludeStack = false }) => super.toString();
+
+  @override
+  DiagnosticsNode describeForError(String name) {
+    return DiagnosticsProperty<Ticker>(name, this, style: DiagnosticsTreeStyle.errorProperty);
+  }
 }
 
 // Forces two frames and checks that:

packages/flutter/test/rendering/viewport_test.dart

@@ -5,6 +5,7 @@
 import 'dart:ui';
 
 import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
@@ -979,4 +980,163 @@ void main() {
     await tester.pumpAndSettle();
     expect(controller.offset, 300.0);
   });
+
+  group('unbounded constraints control test', () {
+    Widget buildNestedWidget([Axis a1 = Axis.vertical, Axis a2 = Axis.horizontal]) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: Container(
+            child: ListView(
+              scrollDirection: a1,
+              children: List<Widget>.generate(10, (int y) {
+                return Container(
+                  child: ListView(
+                    scrollDirection: a2,
+                  ),
+                );
+              }),
+            ),
+          ),
+        ),
+      );
+    }
+
+    Future<void> expectFlutterError({
+      Widget widget,
+      WidgetTester tester,
+      String message,
+    }) async {
+      final List<FlutterErrorDetails> errors = <FlutterErrorDetails>[];
+      final FlutterExceptionHandler oldHandler = FlutterError.onError;
+      FlutterError.onError = (FlutterErrorDetails error) => errors.add(error);
+      try {
+        await tester.pumpWidget(widget);
+      } finally {
+        FlutterError.onError = oldHandler;
+      }
+      expect(errors, isNotEmpty);
+      expect(errors.first.exception, isFlutterError);
+      expect(errors.first.exception.toStringDeep(), message);
+    }
+
+    testWidgets('Horizontal viewport was given unbounded height', (WidgetTester tester) async {
+      await expectFlutterError(
+        widget: buildNestedWidget(),
+        tester: tester,
+        message:
+          'FlutterError\n'
+          '   Horizontal viewport was given unbounded height.\n'
+          '   Viewports expand in the cross axis to fill their container and\n'
+          '   constrain their children to match their extent in the cross axis.\n'
+          '   In this case, a horizontal viewport was given an unlimited amount\n'
+          '   of vertical space in which to expand.\n',
+      );
+    });
+
+    testWidgets('Horizontal viewport was given unbounded width', (WidgetTester tester) async {
+      await expectFlutterError(
+        widget: buildNestedWidget(Axis.horizontal, Axis.horizontal),
+        tester: tester,
+        message:
+          'FlutterError\n'
+          '   Horizontal viewport was given unbounded width.\n'
+          '   Viewports expand in the scrolling direction to fill their\n'
+          '   container.In this case, a horizontal viewport was given an\n'
+          '   unlimited amount of horizontal space in which to expand. This\n'
+          '   situation typically happens when a scrollable widget is nested\n'
+          '   inside another scrollable widget.\n'
+          '   If this widget is always nested in a scrollable widget there is\n'
+          '   no need to use a viewport because there will always be enough\n'
+          '   horizontal space for the children. In this case, consider using a\n'
+          '   Row instead. Otherwise, consider using the "shrinkWrap" property\n'
+          '   (or a ShrinkWrappingViewport) to size the width of the viewport\n'
+          '   to the sum of the widths of its children.\n'
+      );
+    });
+
+    testWidgets('Vertical viewport was given unbounded width', (WidgetTester tester) async {
+      await expectFlutterError(
+        widget: buildNestedWidget(Axis.horizontal, Axis.vertical),
+        tester: tester,
+        message:
+          'FlutterError\n'
+          '   Vertical viewport was given unbounded width.\n'
+          '   Viewports expand in the cross axis to fill their container and\n'
+          '   constrain their children to match their extent in the cross axis.\n'
+          '   In this case, a vertical viewport was given an unlimited amount\n'
+          '   of horizontal space in which to expand.\n'
+      );
+    });
+
+    testWidgets('Vertical viewport was given unbounded height', (WidgetTester tester) async {
+      await expectFlutterError(
+        widget: buildNestedWidget(Axis.vertical, Axis.vertical),
+        tester: tester,
+        message:
+          'FlutterError\n'
+          '   Vertical viewport was given unbounded height.\n'
+          '   Viewports expand in the scrolling direction to fill their\n'
+          '   container. In this case, a vertical viewport was given an\n'
+          '   unlimited amount of vertical space in which to expand. This\n'
+          '   situation typically happens when a scrollable widget is nested\n'
+          '   inside another scrollable widget.\n'
+          '   If this widget is always nested in a scrollable widget there is\n'
+          '   no need to use a viewport because there will always be enough\n'
+          '   vertical space for the children. In this case, consider using a\n'
+          '   Column instead. Otherwise, consider using the "shrinkWrap"\n'
+          '   property (or a ShrinkWrappingViewport) to size the height of the\n'
+          '   viewport to the sum of the heights of its children.\n'
+      );
+    });
+  });
+
+  test('Viewport debugThrowIfNotCheckingIntrinsics() control test', () {
+    final RenderViewport renderViewport = RenderViewport(
+      crossAxisDirection: AxisDirection.right, offset: ViewportOffset.zero()
+    );
+    FlutterError error;
+    try {
+      renderViewport.computeMinIntrinsicHeight(0);
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(
+      error.toStringDeep(),
+      'FlutterError\n'
+      '   RenderViewport does not support returning intrinsic dimensions.\n'
+      '   Calculating the intrinsic dimensions would require instantiating\n'
+      '   every child of the viewport, which defeats the point of viewports\n'
+      '   being lazy.\n'
+      '   If you are merely trying to shrink-wrap the viewport in the main\n'
+      '   axis direction, consider a RenderShrinkWrappingViewport render\n'
+      '   object (ShrinkWrappingViewport widget), which achieves that\n'
+      '   effect without implementing the intrinsic dimension API.\n',
+    );
+
+    final RenderShrinkWrappingViewport renderShrinkWrappingViewport = RenderShrinkWrappingViewport(
+      crossAxisDirection: AxisDirection.right, offset: ViewportOffset.zero()
+    );
+    error = null;
+    try {
+      renderShrinkWrappingViewport.computeMinIntrinsicHeight(0);
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(
+      error.toStringDeep(),
+      'FlutterError\n'
+      '   RenderShrinkWrappingViewport does not support returning intrinsic\n'
+      '   dimensions.\n'
+      '   Calculating the intrinsic dimensions would require instantiating\n'
+      '   every child of the viewport, which defeats the point of viewports\n'
+      '   being lazy.\n'
+      '   If you are merely trying to shrink-wrap the viewport in the main\n'
+      '   axis direction, you should be able to achieve that effect by just\n'
+      '   giving the viewport loose constraints, without needing to measure\n'
+      '   its intrinsic dimensions.\n',
+    );
+  });
 }

packages/flutter/test/scheduler/ticker_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -24,7 +25,26 @@ void main() {
     expect(ticker.isActive, isTrue);
     expect(tickCount, equals(0));
 
-    expect(ticker.start, throwsFlutterError);
+    FlutterError error;
+    try {
+      ticker.start();
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error.diagnostics.length, 3);
+    expect(error.diagnostics.last, isInstanceOf<DiagnosticsProperty<Ticker>>());
+    expect(
+      error.toStringDeep(),
+      startsWith(
+        'FlutterError\n'
+        '   A ticker was started twice.\n'
+        '   A ticker that is already active cannot be started again without\n'
+        '   first stopping it.\n'
+        '   The affected ticker was:\n'
+        '     Ticker()\n',
+      ),
+    );
 
     await tester.pump(const Duration(milliseconds: 10));
 

packages/flutter/test/services/asset_bundle_test.dart

@@ -56,4 +56,24 @@ void main() {
     expect(key.name, 'one');
     expect(key.scale, 1.0);
   });
+
+  test('NetworkAssetBundle control test', () async {
+    final Uri uri = Uri.http('example.org', '/path');
+    final NetworkAssetBundle bundle = NetworkAssetBundle(uri);
+    FlutterError error;
+    try {
+      await bundle.load('key');
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error.diagnostics.length, 2);
+    expect(error.diagnostics.last, isInstanceOf<IntProperty>());
+    expect(
+      error.toStringDeep(),
+      'FlutterError\n'
+      '   Unable to load asset: key\n'
+      '   HTTP status code: 404\n',
+    );
+  }, skip: true);
 }

packages/flutter/test/widgets/animated_list_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/src/foundation/diagnostics.dart';
 import 'package:flutter/widgets.dart';
 
 void main() {
@@ -319,4 +320,47 @@ void main() {
       expect(tester.getTopLeft(find.text('item 0')).dy, 200);
     });
   });
+
+  testWidgets('AnimatedList.of() called with a context that does not contain AnimatedList',
+    (WidgetTester tester) async {
+    final GlobalKey key = GlobalKey();
+    await tester.pumpWidget(Container(key: key));
+    FlutterError error;
+    try {
+      AnimatedList.of(key.currentContext);
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error.diagnostics.length, 4);
+    expect(error.diagnostics[2].level, DiagnosticLevel.hint);
+    expect(
+      error.diagnostics[2].toStringDeep(),
+      equalsIgnoringHashCodes(
+        'This can happen when the context provided is from the same\n'
+        'StatefulWidget that built the AnimatedList. Please see the\n'
+        'AnimatedList documentation for examples of how to refer to an\n'
+        'AnimatedListState object:\n'
+        'https://api.flutter.dev/flutter/widgets/AnimatedListState-class.html\n'
+      ),
+    );
+    expect(error.diagnostics[3], isInstanceOf<DiagnosticsProperty<Element>>());
+    expect(
+      error.toStringDeep(),
+      equalsIgnoringHashCodes(
+        'FlutterError\n'
+        '   AnimatedList.of() called with a context that does not contain an\n'
+        '   AnimatedList.\n'
+        '   No AnimatedList ancestor could be found starting from the context\n'
+        '   that was passed to AnimatedList.of().\n'
+        '   This can happen when the context provided is from the same\n'
+        '   StatefulWidget that built the AnimatedList. Please see the\n'
+        '   AnimatedList documentation for examples of how to refer to an\n'
+        '   AnimatedListState object:\n'
+        '   https://api.flutter.dev/flutter/widgets/AnimatedListState-class.html\n'
+        '   The context used was:\n'
+        '     Container-[GlobalKey#32cc6]\n'
+      ),
+    );
+  });
 }

packages/flutter/test/widgets/app_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -19,4 +20,72 @@ void main() {
     );
     expect(find.byKey(key), findsOneWidget);
   });
+
+  group('error control test', () {
+    Future<void> expectFlutterError({
+      GlobalKey<NavigatorState> key,
+      Widget widget,
+      WidgetTester tester,
+      String errorMessage,
+    }) async {
+      await tester.pumpWidget(widget);
+      FlutterError error;
+      try {
+        key.currentState.pushNamed('/path');
+      } on FlutterError catch (e) {
+        error = e;
+      } finally {
+        expect(error, isNotNull);
+        expect(error, isFlutterError);
+        expect(error.toStringDeep(), errorMessage);
+      }
+    }
+
+    testWidgets('push unknown route when onUnknownRoute is null', (WidgetTester tester) async {
+      final GlobalKey<NavigatorState> key = GlobalKey<NavigatorState>();
+      expectFlutterError(
+        key: key,
+        tester: tester,
+        widget: MaterialApp(
+          navigatorKey: key,
+          home: Container(),
+          onGenerateRoute: (_) => null,
+        ),
+        errorMessage:
+          'FlutterError\n'
+          '   Could not find a generator for route RouteSettings("/path", null)\n'
+          '   in the _WidgetsAppState.\n'
+          '   Generators for routes are searched for in the following order:\n'
+          '    1. For the "/" route, the "home" property, if non-null, is used.\n'
+          '    2. Otherwise, the "routes" table is used, if it has an entry for\n'
+          '   the route.\n'
+          '    3. Otherwise, onGenerateRoute is called. It should return a\n'
+          '   non-null value for any valid route not handled by "home" and\n'
+          '   "routes".\n'
+          '    4. Finally if all else fails onUnknownRoute is called.\n'
+          '   Unfortunately, onUnknownRoute was not set.\n',
+      );
+    });
+
+    testWidgets('push unknown route when onUnknownRoute returns null', (WidgetTester tester) async {
+      final GlobalKey<NavigatorState> key = GlobalKey<NavigatorState>();
+      expectFlutterError(
+        key: key,
+        tester: tester,
+        widget: MaterialApp(
+          navigatorKey: key,
+          home: Container(),
+          onGenerateRoute: (_) => null,
+          onUnknownRoute: (_) => null,
+        ),
+        errorMessage:
+          'FlutterError\n'
+          '   The onUnknownRoute callback returned null.\n'
+          '   When the _WidgetsAppState requested the route\n'
+          '   RouteSettings("/path", null) from its onUnknownRoute callback,\n'
+          '   the callback returned null. Such callbacks must never return\n'
+          '   null.\n' ,
+      );
+    });
+  });
 }

packages/flutter/test/widgets/binding_test.dart

@@ -138,4 +138,36 @@ void main() {
     expect(tester.binding.hasScheduledFrame, isFalse);
     expect(frameCount, 1);
   });
+
+  testWidgets('scheduleFrameCallback error control test', (WidgetTester tester) async {
+    FlutterError error;
+    try {
+      tester.binding.scheduleFrameCallback(null, rescheduling: true);
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error.diagnostics.length, 3);
+    expect(error.diagnostics.last.level, DiagnosticLevel.hint);
+    expect(
+      error.diagnostics.last.toStringDeep(),
+      equalsIgnoringHashCodes(
+        'If this is the initial registration of the callback, or if the\n'
+        'callback is asynchronous, then do not use the "rescheduling"\n'
+        'argument.\n'
+      ),
+    );
+    expect(
+      error.toStringDeep(),
+      'FlutterError\n'
+      '   scheduleFrameCallback called with rescheduling true, but no\n'
+      '   callback is in scope.\n'
+      '   The "rescheduling" argument should only be set to true if the\n'
+      '   callback is being reregistered from within the callback itself,\n'
+      '   and only then if the callback itself is entirely synchronous.\n'
+      '   If this is the initial registration of the callback, or if the\n'
+      '   callback is asynchronous, then do not use the "rescheduling"\n'
+      '   argument.\n'
+    );
+  });
 }

packages/flutter/test/widgets/container_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
+import 'package:mockito/mockito.dart';
 
 import '../rendering/mock_canvas.dart';
 
@@ -461,6 +462,32 @@ void main() {
         '                   android)\n',
       ),
     );
+
+    final RenderBox decoratedBox = tester.renderObject(find.byType(DecoratedBox).last);
+    final PaintingContext context = _MockPaintingContext();
+    final Canvas canvas = _MockCanvas();
+    int saveCount = 0;
+    when(canvas.getSaveCount()).thenAnswer((_) => saveCount++);
+    when(context.canvas).thenReturn(canvas);
+    FlutterError error;
+    try {
+      decoratedBox.paint(context, const Offset(0, 0));
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(
+      error.toStringDeep(),
+      'FlutterError\n'
+      '   BoxDecoration painter had mismatching save and restore calls.\n'
+      '   Before painting the decoration, the canvas save count was 0.\n'
+      '   After painting it, the canvas save count was 2. Every call to\n'
+      '   save() or saveLayer() must be matched by a call to restore().\n'
+      '   The decoration was:\n'
+      '     BoxDecoration(color: Color(0xffffff00))\n'
+      '   The painter was:\n'
+      '     BoxPainter for BoxDecoration(color: Color(0xffffff00))\n'
+    );
   });
 
   testWidgets('Can be placed in an infinite box', (WidgetTester tester) async {
@@ -472,3 +499,6 @@ void main() {
     );
   });
 }
+
+class _MockPaintingContext extends Mock implements PaintingContext {}
+class _MockCanvas extends Mock implements Canvas {}

packages/flutter/test/widgets/custom_multi_child_layout_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
@@ -90,6 +91,86 @@ class NotifierLayoutDelegate extends MultiChildLayoutDelegate {
   }
 }
 
+// LayoutDelegate that lays out child with id 0 and 1
+// Used in the 'performLayout error control test' test case to trigger:
+//  - error when laying out a non existent child and a child that has not been laid out
+class ZeroAndOneIdLayoutDelegate extends MultiChildLayoutDelegate {
+  @override
+  void performLayout(Size size) {
+    final BoxConstraints constraints = BoxConstraints.loose(size);
+    layoutChild(0, constraints);
+    layoutChild(1, constraints);
+  }
+
+  @override
+  bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) => true;
+}
+
+// Used in the 'performLayout error control test' test case
+//  to trigger an error when laying out child more than once
+class DuplicateLayoutDelegate extends MultiChildLayoutDelegate {
+  @override
+  void performLayout(Size size) {
+    final BoxConstraints constraints = BoxConstraints.loose(size);
+    layoutChild(0, constraints);
+    layoutChild(0, constraints);
+  }
+
+  @override
+  bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) => true;
+}
+// Used in the 'performLayout error control test' test case
+//  to trigger an error when positioning non existent child
+class NonExistentPositionDelegate extends MultiChildLayoutDelegate {
+  @override
+  void performLayout(Size size) {
+    positionChild(0, const Offset(0, 0));
+    positionChild(1, const Offset(0, 0));
+  }
+
+  @override
+  bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) => true;
+}
+
+// Used in the 'performLayout error control test' test case
+//  to trigger an error when positioning with null offset
+class NullOffsetPositionDelegate extends MultiChildLayoutDelegate {
+  @override
+  void performLayout(Size size) {
+    positionChild(0, null);
+  }
+
+  @override
+  bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) => true;
+}
+
+// Used in the 'performLayout error control test' test case for triggering
+//  to layout child more than once
+class InvalidConstraintsChildLayoutDelegate extends MultiChildLayoutDelegate {
+  @override
+  void performLayout(Size size) {
+    final BoxConstraints constraints = BoxConstraints.loose(
+      // Invalid because width and height must be greater than or equal to 0
+      const Size(-1, -1)
+    );
+    layoutChild(0, constraints);
+  }
+
+  @override
+  bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) => true;
+}
+
+class LayoutWithMissingId extends ParentDataWidget<CustomMultiChildLayout> {
+  const LayoutWithMissingId({
+    Key key,
+    @required Widget child,
+  }) : assert(child != null),
+       super(key: key, child: child);
+
+  @override
+  void applyParentData(RenderObject renderObject) {}
+}
+
 void main() {
   testWidgets('Control test for CustomMultiChildLayout', (WidgetTester tester) async {
     final TestMultiChildLayoutDelegate delegate = TestMultiChildLayoutDelegate();
@@ -199,4 +280,170 @@ void main() {
     box = tester.renderObject(find.byType(CustomMultiChildLayout));
     expect(box.size, equals(const Size(150.0, 240.0)));
   });
+
+  group('performLayout error control test', () {
+    Widget buildSingleChildFrame(MultiChildLayoutDelegate delegate) {
+      return Center(
+        child: CustomMultiChildLayout(
+          children: <Widget>[LayoutId(id: 0, child: const SizedBox())],
+          delegate: delegate,
+        ),
+      );
+    }
+
+    Future<void> expectFlutterErrorMessage({
+      Widget widget,
+      MultiChildLayoutDelegate delegate,
+      @required WidgetTester tester,
+      @required String message,
+    }) async {
+      final FlutterExceptionHandler oldHandler = FlutterError.onError;
+      final List<FlutterErrorDetails> errors = <FlutterErrorDetails>[];
+      FlutterError.onError = (FlutterErrorDetails error) => errors.add(error);
+      try {
+        await tester.pumpWidget(widget ?? buildSingleChildFrame(delegate));
+      } finally {
+        FlutterError.onError = oldHandler;
+      }
+      expect(errors.length, isNonZero);
+      expect(errors.first, isNotNull);
+      expect(errors.first.exception, isFlutterError);
+      expect(errors.first.exception.toStringDeep(), equalsIgnoringHashCodes(message));
+    }
+
+    testWidgets('layoutChild on non existent child', (WidgetTester tester) async {
+      expectFlutterErrorMessage(
+        tester: tester,
+        delegate: ZeroAndOneIdLayoutDelegate(),
+        message:
+          'FlutterError\n'
+          '   The ZeroAndOneIdLayoutDelegate custom multichild layout delegate\n'
+          '   tried to lay out a non-existent child.\n'
+          '   There is no child with the id "1".\n'
+      );
+    });
+
+    testWidgets('layoutChild more than once', (WidgetTester tester) async {
+      expectFlutterErrorMessage(
+          tester: tester,
+          delegate: DuplicateLayoutDelegate(),
+          message:
+            'FlutterError\n'
+            '   The DuplicateLayoutDelegate custom multichild layout delegate\n'
+            '   tried to lay out the child with id "0" more than once.\n'
+            '   Each child must be laid out exactly once.\n'
+      );
+    });
+
+    testWidgets('layoutChild on invalid size constraint', (WidgetTester tester) async {
+      expectFlutterErrorMessage(
+        tester: tester,
+        delegate: InvalidConstraintsChildLayoutDelegate(),
+        message:
+          'FlutterError\n'
+          '   The InvalidConstraintsChildLayoutDelegate custom multichild\n'
+          '   layout delegate provided invalid box constraints for the child\n'
+          '   with id "0".\n'
+          '   FlutterError\n'
+          '   The minimum width and height must be greater than or equal to\n'
+          '   zero.\n'
+          '   The maximum width must be greater than or equal to the minimum\n'
+          '   width.\n'
+          '   The maximum height must be greater than or equal to the minimum\n'
+          '   height.\n'
+      );
+    });
+
+    testWidgets('positionChild on non existent child', (WidgetTester tester) async {
+      expectFlutterErrorMessage(
+        tester: tester,
+        delegate: NonExistentPositionDelegate(),
+        message:
+          'FlutterError\n'
+          '   The NonExistentPositionDelegate custom multichild layout delegate\n'
+          '   tried to position out a non-existent child:\n'
+          '   There is no child with the id "1".\n'
+      );
+    });
+
+    testWidgets('positionChild on non existent child', (WidgetTester tester) async {
+      expectFlutterErrorMessage(
+        tester: tester,
+        delegate: NullOffsetPositionDelegate(),
+        message:
+          'FlutterError\n'
+          '   The NullOffsetPositionDelegate custom multichild layout delegate\n'
+          '   provided a null position for the child with id "0".\n',
+      );
+    });
+
+    testWidgets("_callPerformLayout on child that doesn't have id", (WidgetTester tester) async {
+      expectFlutterErrorMessage(
+        widget: Center(
+          child: CustomMultiChildLayout(
+            children: <Widget>[LayoutWithMissingId(child: Container(width: 100))],
+            delegate: PreferredSizeDelegate(preferredSize: const Size(10, 10)),
+          ),
+        ),
+        tester: tester,
+        message:
+          'FlutterError\n'
+          '   Every child of a RenderCustomMultiChildLayoutBox must have an ID\n'
+          '   in its parent data.\n'
+          '   The following child has no ID: RenderConstrainedBox#00000 NEEDS-LAYOUT NEEDS-PAINT:\n'
+          '     creator: ConstrainedBox ← Container ← LayoutWithMissingId ←\n'
+          '       CustomMultiChildLayout ← Center ← [root]\n'
+          '     parentData: offset=Offset(0.0, 0.0); id=null\n'
+          '     constraints: MISSING\n'
+          '     size: MISSING\n'
+          '     additionalConstraints: BoxConstraints(w=100.0, 0.0<=h<=Infinity)\n'
+      );
+    });
+
+    testWidgets('performLayout did not layout a child', (WidgetTester tester) async {
+      expectFlutterErrorMessage(
+        widget: Center(
+          child: CustomMultiChildLayout(
+            children: <Widget>[
+              LayoutId(id: 0, child: Container(width: 100)),
+              LayoutId(id: 1, child: Container(width: 100)),
+              LayoutId(id: 2, child: Container(width: 100)),
+            ],
+            delegate: ZeroAndOneIdLayoutDelegate(),
+          ),
+        ),
+        tester: tester,
+        message:
+          'FlutterError\n'
+          '   Each child must be laid out exactly once.\n'
+          '   The ZeroAndOneIdLayoutDelegate custom multichild layout delegate'
+          ' forgot to lay out the following child:\n'
+          '     2: RenderConstrainedBox#62a34 NEEDS-LAYOUT NEEDS-PAINT\n'
+      );
+    });
+
+    testWidgets('performLayout did not layout multiple child', (WidgetTester tester) async {
+      expectFlutterErrorMessage(
+        widget: Center(
+          child: CustomMultiChildLayout(
+            children: <Widget>[
+              LayoutId(id: 0, child: Container(width: 100)),
+              LayoutId(id: 1, child: Container(width: 100)),
+              LayoutId(id: 2, child: Container(width: 100)),
+              LayoutId(id: 3, child: Container(width: 100)),
+            ],
+            delegate: ZeroAndOneIdLayoutDelegate(),
+          ),
+        ),
+        tester: tester,
+        message:
+          'FlutterError\n'
+          '   Each child must be laid out exactly once.\n'
+          '   The ZeroAndOneIdLayoutDelegate custom multichild layout delegate'
+          ' forgot to lay out the following children:\n'
+          '     2: RenderConstrainedBox#62a34 NEEDS-LAYOUT NEEDS-PAINT\n'
+          '     3: RenderConstrainedBox#62a34 NEEDS-LAYOUT NEEDS-PAINT\n'
+      );
+    });
+  });
 }

packages/flutter/test/widgets/custom_paint_test.dart

@@ -2,9 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:mockito/mockito.dart';
 
 class TestCustomPainter extends CustomPainter {
   TestCustomPainter({ this.log, this.name });
@@ -21,6 +22,25 @@ class TestCustomPainter extends CustomPainter {
   bool shouldRepaint(TestCustomPainter oldPainter) => true;
 }
 
+class TestCustomPainterWithCustomSemanticsBuilder extends TestCustomPainter {
+  TestCustomPainterWithCustomSemanticsBuilder() : super(log: <String>[]);
+
+  @override
+  SemanticsBuilderCallback get semanticsBuilder => (Size size) {
+    const Key key = Key('0');
+    const Rect rect = Rect.fromLTRB(0, 0, 0, 0);
+    const SemanticsProperties semanticsProperties = SemanticsProperties();
+    return <CustomPainterSemantics>[
+      const CustomPainterSemantics(key: key, rect: rect, properties: semanticsProperties),
+      const CustomPainterSemantics(key: key, rect: rect, properties: semanticsProperties),
+    ];
+  };
+}
+
+class MockCanvas extends Mock implements Canvas {}
+
+class MockPaintingContext extends Mock implements PaintingContext {}
+
 void main() {
   testWidgets('Control test for custom painting', (WidgetTester tester) async {
     final List<String> log = <String>[];
@@ -44,6 +64,108 @@ void main() {
     expect(log, equals(<String>['background', 'child', 'foreground']));
   });
 
+  testWidgets('Throws FlutterError on custom painter incorrect restore/save calls', (
+      WidgetTester tester) async {
+    final GlobalKey target = GlobalKey();
+    final List<String> log = <String>[];
+    await tester.pumpWidget(CustomPaint(
+      key: target,
+      isComplex: true,
+      painter: TestCustomPainter(log: log),
+    ));
+    final RenderCustomPaint renderCustom = target.currentContext.findRenderObject();
+    final Canvas canvas = MockCanvas();
+    int saveCount = 0;
+    when(canvas.getSaveCount()).thenAnswer((_) => saveCount++);
+    final PaintingContext paintingContext = MockPaintingContext();
+    when(paintingContext.canvas).thenReturn(canvas);
+
+    FlutterError getError() {
+      FlutterError error;
+      try {
+        renderCustom.paint(paintingContext, const Offset(0, 0));
+      } on FlutterError catch (e) {
+        error = e;
+      }
+      return error;
+    }
+
+    FlutterError error = getError();
+    expect(error.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   The TestCustomPainter#00000() custom painter called canvas.save()\n'
+      '   or canvas.saveLayer() at least 1 more time than it called\n'
+      '   canvas.restore().\n'
+      '   This leaves the canvas in an inconsistent state and will probably\n'
+      '   result in a broken display.\n'
+      '   You must pair each call to save()/saveLayer() with a later\n'
+      '   matching call to restore().\n'
+    ));
+
+    when(canvas.getSaveCount()).thenAnswer((_) => saveCount--);
+    error = getError();
+    expect(error.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   The TestCustomPainter#00000() custom painter called\n'
+      '   canvas.restore() 1 more time than it called canvas.save() or\n'
+      '   canvas.saveLayer().\n'
+      '   This leaves the canvas in an inconsistent state and will result\n'
+      '   in a broken display.\n'
+      '   You should only call restore() if you first called save() or\n'
+      '   saveLayer().\n'
+    ));
+
+    when(canvas.getSaveCount()).thenAnswer((_) => saveCount += 2);
+    error = getError();
+    expect(error.toStringDeep(), contains('2 more times'));
+
+    when(canvas.getSaveCount()).thenAnswer((_) => saveCount -= 2);
+    error = getError();
+    expect(error.toStringDeep(), contains('2 more times'));
+  });
+
+  testWidgets('assembleSemanticsNode throws FlutterError', (WidgetTester tester) async {
+    final List<String> log = <String>[];
+    final GlobalKey target = GlobalKey();
+    await tester.pumpWidget(CustomPaint(
+      key: target,
+      isComplex: true,
+      painter: TestCustomPainter(log: log),
+    ));
+    final RenderCustomPaint renderCustom = target.currentContext.findRenderObject();
+    FlutterError error;
+    try {
+      renderCustom.assembleSemanticsNode(
+        null,
+        null,
+        <SemanticsNode>[SemanticsNode()],
+      );
+    } on FlutterError catch (e) {
+      error = e;
+    }
+    expect(error, isNotNull);
+    expect(error.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   RenderCustomPaint does not have a child widget but received a\n'
+      '   non-empty list of child SemanticsNode:\n'
+      '   SemanticsNode#1(Rect.fromLTRB(0.0, 0.0, 0.0, 0.0), invisible)\n'
+    ));
+
+    await tester.pumpWidget(CustomPaint(
+      key: target,
+      isComplex: true,
+      painter: TestCustomPainterWithCustomSemanticsBuilder(),
+    ));
+    final dynamic exception = tester.takeException();
+    expect(exception, isFlutterError);
+    error = exception;
+    expect(error.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   Failed to update the list of CustomPainterSemantics:\n'
+      '   - duplicate key [<\'0\'>] found at position 1\n'
+    ));
+  });
+
   testWidgets('CustomPaint sizing', (WidgetTester tester) async {
     final GlobalKey target = GlobalKey();
 

packages/flutter/test/widgets/debug_test.dart

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('debugChildrenHaveDuplicateKeys control test', () {
+    const Key key = Key('key');
+    final List<Widget> children = <Widget>[
+      Container(key: key),
+      Container(key: key),
+    ];
+    final Widget widget = Flex(
+      direction: Axis.vertical,
+      children: children
+    );
+    FlutterError error;
+    try {
+      debugChildrenHaveDuplicateKeys(widget, children);
+    } on FlutterError catch (e) {
+      error = e;
+    } finally {
+      expect(error, isNotNull);
+      expect(
+        error.toStringDeep(),
+        equalsIgnoringHashCodes(
+          'FlutterError\n'
+          '   Duplicate keys found.\n'
+          '   If multiple keyed nodes exist as children of another node, they\n'
+          '   must have unique keys.\n'
+          '   Flex(direction: vertical, mainAxisAlignment: start,\n'
+          '   crossAxisAlignment: center) has multiple children with key\n'
+          '   [<\'key\'>].\n',
+        ),
+      );
+    }
+  });
+
+  test('debugItemsHaveDuplicateKeys control test', () {
+    const Key key = Key('key');
+    final List<Widget> items = <Widget>[
+      Container(key: key),
+      Container(key: key),
+    ];
+    FlutterError error;
+    try {
+      debugItemsHaveDuplicateKeys(items);
+    } on FlutterError catch (e) {
+      error = e;
+    } finally {
+      expect(error, isNotNull);
+      expect(
+        error.toStringDeep(),
+        equalsIgnoringHashCodes(
+          'FlutterError\n'
+          '   Duplicate key found: [<\'key\'>].\n'
+        ),
+      );
+    }
+  });
+
+  testWidgets('debugCheckHasTable control test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Builder(
+        builder: (BuildContext context) {
+          FlutterError error;
+          try {
+            debugCheckHasTable(context);
+          } on FlutterError catch (e) {
+            error = e;
+          } finally {
+            expect(error, isNotNull);
+            expect(error.diagnostics.length, 4);
+            expect(error.diagnostics[2], isInstanceOf<DiagnosticsProperty<Element>>());
+            expect(
+              error.toStringDeep(),
+              equalsIgnoringHashCodes(
+                'FlutterError\n'
+                '   No Table widget found.\n'
+                '   Builder widgets require a Table widget ancestor.\n'
+                '   The specific widget that could not find a Table ancestor was:\n'
+                '     Builder\n'
+                '   The ownership chain for the affected widget is: "Builder ←\n'
+                '     [root]"\n'
+              ),
+            );
+          }
+          return Container();
+        }
+      ),
+    );
+  });
+
+  testWidgets('debugCheckHasMediaQuery control test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Builder(
+        builder: (BuildContext context) {
+          FlutterError error;
+          try {
+            debugCheckHasMediaQuery(context);
+          } on FlutterError catch (e) {
+            error = e;
+          } finally {
+            expect(error, isNotNull);
+            expect(error.diagnostics.length, 5);
+            expect(error.diagnostics[2], isInstanceOf<DiagnosticsProperty<Element>>());
+            expect(error.diagnostics.last.level, DiagnosticLevel.hint);
+            expect(
+              error.diagnostics.last.toStringDeep(),
+              equalsIgnoringHashCodes(
+                'Typically, the MediaQuery widget is introduced by the MaterialApp\n'
+                'or WidgetsApp widget at the top of your application widget tree.\n'
+              ),
+            );
+            expect(
+              error.toStringDeep(),
+              equalsIgnoringHashCodes(
+                'FlutterError\n'
+                '   No MediaQuery widget found.\n'
+                '   Builder widgets require a MediaQuery widget ancestor.\n'
+                '   The specific widget that could not find a MediaQuery ancestor\n'
+                '   was:\n'
+                '     Builder\n'
+                '   The ownership chain for the affected widget is: "Builder ←\n'
+                '     [root]"\n'
+                '   Typically, the MediaQuery widget is introduced by the MaterialApp\n'
+                '   or WidgetsApp widget at the top of your application widget tree.\n'
+              ),
+            );
+          }
+          return Container();
+        }
+      ),
+    );
+  });
+
+  test('debugWidgetBuilderValue control test', () {
+    final Widget widget = Container();
+    FlutterError error;
+    try {
+      debugWidgetBuilderValue(widget, null);
+    } on FlutterError catch (e) {
+      error = e;
+    } finally {
+      expect(error, isNotNull);
+      expect(error.diagnostics.length, 4);
+      expect(error.diagnostics[1], isInstanceOf<DiagnosticsProperty<Widget>>());
+      expect(error.diagnostics[1].style, DiagnosticsTreeStyle.errorProperty);
+      expect(
+        error.diagnostics[1].toStringDeep(),
+        equalsIgnoringHashCodes(
+          'The offending widget is:\n'
+          '  Container\n'
+        )
+      );
+      expect(error.diagnostics[2].level, DiagnosticLevel.info);
+      expect(error.diagnostics[3].level, DiagnosticLevel.hint);
+      expect(
+        error.diagnostics[3].toStringDeep(),
+        equalsIgnoringHashCodes(
+          'To return an empty space that causes the building widget to fill\n'
+          'available room, return "Container()". To return an empty space\n'
+          'that takes as little room as possible, return "Container(width:\n'
+          '0.0, height: 0.0)".\n',
+        )
+      );
+      expect(
+        error.toStringDeep(),
+        equalsIgnoringHashCodes(
+          'FlutterError\n'
+          '   A build function returned null.\n'
+          '   The offending widget is:\n'
+          '     Container\n'
+          '   Build functions must never return null.\n'
+          '   To return an empty space that causes the building widget to fill\n'
+          '   available room, return "Container()". To return an empty space\n'
+          '   that takes as little room as possible, return "Container(width:\n'
+          '   0.0, height: 0.0)".\n'
+        ),
+      );
+      error = null;
+    }
+    try {
+      debugWidgetBuilderValue(widget, widget);
+    } on FlutterError catch (e) {
+      error = e;
+    } finally {
+      expect(error, isNotNull);
+      expect(error.diagnostics.length, 3);
+      expect(error.diagnostics[1], isInstanceOf<DiagnosticsProperty<Widget>>());
+      expect(error.diagnostics[1].style, DiagnosticsTreeStyle.errorProperty);
+      expect(
+        error.diagnostics[1].toStringDeep(),
+        equalsIgnoringHashCodes(
+          'The offending widget is:\n'
+          '  Container\n'
+        )
+      );
+      expect(
+        error.toStringDeep(),
+        equalsIgnoringHashCodes(
+          'FlutterError\n'
+          '   A build function returned context.widget.\n'
+          '   The offending widget is:\n'
+          '     Container\n'
+          '   Build functions must never return their BuildContext parameter\'s\n'
+          '   widget or a child that contains "context.widget". Doing so\n'
+          '   introduces a loop in the widget tree that can cause the app to\n'
+          '   crash.\n'
+        ),
+      );
+    }
+  });
+
+  test('debugAssertAllWidgetVarsUnset', () {
+    debugHighlightDeprecatedWidgets = true;
+    FlutterError error;
+    try {
+      debugAssertAllWidgetVarsUnset('The value of a widget debug variable was changed by the test.');
+    } on FlutterError catch (e) {
+      error = e;
+    } finally {
+      expect(error, isNotNull);
+      expect(error.diagnostics.length, 1);
+      expect(
+        error.toStringDeep(),
+        'FlutterError\n'
+        '   The value of a widget debug variable was changed by the test.\n',
+      );
+    }
+  });
+}

packages/flutter/test/widgets/dismissible_test.dart

@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/gestures.dart' show DragStartBehavior;
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
-import 'package:flutter/gestures.dart' show DragStartBehavior;
 
 const double itemExtent = 100.0;
 Axis scrollDirection = Axis.vertical;
@@ -732,4 +732,68 @@ void main() {
     expect(dismissedItems, isEmpty);
     expect(confirmDismissDirection, DismissDirection.endToStart);
   });
+
+  testWidgets('setState that does not remove the Dismissible from tree should throws Error', (WidgetTester tester) async {
+    scrollDirection = Axis.vertical;
+    dismissDirection = DismissDirection.horizontal;
+
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: StatefulBuilder(
+        builder: (BuildContext context, StateSetter setState) {
+          return ListView(
+            dragStartBehavior: DragStartBehavior.down,
+            scrollDirection: scrollDirection,
+            itemExtent: itemExtent,
+            children: <Widget>[
+              Dismissible(
+                dragStartBehavior: DragStartBehavior.down,
+                key: const ValueKey<int>(1),
+                direction: dismissDirection,
+                onDismissed: (DismissDirection direction) {
+                  setState(() {
+                    reportedDismissDirection = direction;
+                    expect(dismissedItems.contains(1), isFalse);
+                    dismissedItems.add(1);
+                  });
+                },
+                background: background,
+                dismissThresholds: const <DismissDirection, double>{},
+                crossAxisEndOffset: crossAxisEndOffset,
+                child: Container(
+                  width: itemExtent,
+                  height: itemExtent,
+                  child: Text(1.toString()),
+                ),
+              ),
+            ]
+          );
+        },
+      ),
+    ));
+    expect(dismissedItems, isEmpty);
+    await dismissItem(tester, 1, gestureDirection: AxisDirection.right);
+    expect(dismissedItems, equals(<int>[1]));
+    final dynamic exception =  tester.takeException();
+    expect(exception, isNotNull);
+    expect(exception, isFlutterError);
+    final FlutterError error = exception;
+    expect(error.diagnostics.last.level, DiagnosticLevel.hint);
+    expect(
+      error.diagnostics.last.toStringDeep(),
+      equalsIgnoringHashCodes(
+        'Make sure to implement the onDismissed handler and to immediately\n'
+        'remove the Dismissible widget from the application once that\n'
+        'handler has fired.\n',
+      ),
+    );
+    expect(
+      error.toStringDeep(),
+      'FlutterError\n'
+      '   A dismissed Dismissible widget is still part of the tree.\n'
+      '   Make sure to implement the onDismissed handler and to immediately\n'
+      '   remove the Dismissible widget from the application once that\n'
+      '   handler has fired.\n',
+    );
+  });
 }

packages/flutter/test/widgets/flow_test.dart

@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter_test/flutter_test.dart';
-import 'package:flutter/widgets.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
 
 class TestFlowDelegate extends FlowDelegate {
   TestFlowDelegate({this.startOffset}) : super(repaint: startOffset);
@@ -45,6 +45,21 @@ class OpacityFlowDelegate extends FlowDelegate {
   bool shouldRepaint(OpacityFlowDelegate oldDelegate) => opacity != oldDelegate.opacity;
 }
 
+// OpacityFlowDelegate that paints one of its children twice
+class DuplicatePainterOpacityFlowDelegate extends OpacityFlowDelegate {
+  DuplicatePainterOpacityFlowDelegate(double opacity) : super(opacity);
+
+  @override
+  void paintChildren(FlowPaintingContext context) {
+    for (int i = 0; i < context.childCount; ++i) {
+      context.paintChild(i, opacity: opacity);
+    }
+    if (context.childCount > 0) {
+      context.paintChild(0, opacity: opacity);
+    }
+  }
+}
+
 void main() {
   testWidgets('Flow control test', (WidgetTester tester) async {
     final AnimationController startOffset = AnimationController.unbounded(
@@ -100,6 +115,28 @@ void main() {
     expect(log, equals(<int>[0]));
   });
 
+  testWidgets('paintChild gets called twice', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Flow(
+        delegate: DuplicatePainterOpacityFlowDelegate(1.0),
+        children: <Widget>[
+          Container(width: 100.0, height: 100.0),
+          Container(width: 100.0, height: 100.0),
+        ],
+      ),
+    );
+    final dynamic exception = tester.takeException();
+    expect(exception, isFlutterError);
+    final FlutterError error = exception;
+    expect(error.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   Cannot call paintChild twice for the same child.\n'
+      '   The flow delegate of type DuplicatePainterOpacityFlowDelegate\n'
+      '   attempted to paint child 0 multiple times, which is not\n'
+      '   permitted.\n'
+    ));
+  });
+
   testWidgets('Flow opacity layer', (WidgetTester tester) async {
     const double opacity = 0.2;
     await tester.pumpWidget(

packages/flutter/test/widgets/framework_test.dart

@@ -657,6 +657,59 @@ void main() {
     );
     test.includeChild = false;
   });
+
+  testWidgets('scheduleBuild while debugBuildingDirtyElements is true', (WidgetTester tester) async {
+    /// ignore here is required for testing purpose because changing the flag properly is hard
+    // ignore: invalid_use_of_protected_member
+    tester.binding.debugBuildingDirtyElements = true;
+    FlutterError error;
+    try {
+      tester.binding.buildOwner.scheduleBuildFor(
+        DirtyElementWithCustomBuildOwner(tester.binding.buildOwner, Container()));
+    } on FlutterError catch (e) {
+      error = e;
+    } finally {
+      expect(error, isNotNull);
+      expect(error.diagnostics.length, 3);
+      expect(error.diagnostics.last.level, DiagnosticLevel.hint);
+      expect(
+        error.diagnostics.last.toStringDeep(),
+        equalsIgnoringHashCodes(
+          'This might be because setState() was called from a layout or\n'
+          'paint callback. If a change is needed to the widget tree, it\n'
+          'should be applied as the tree is being built. Scheduling a change\n'
+          'for the subsequent frame instead results in an interface that\n'
+          'lags behind by one frame. If this was done to make your build\n'
+          'dependent on a size measured at layout time, consider using a\n'
+          'LayoutBuilder, CustomSingleChildLayout, or\n'
+          'CustomMultiChildLayout. If, on the other hand, the one frame\n'
+          'delay is the desired effect, for example because this is an\n'
+          'animation, consider scheduling the frame in a post-frame callback\n'
+          'using SchedulerBinding.addPostFrameCallback or using an\n'
+          'AnimationController to trigger the animation.\n',
+        ),
+      );
+      expect(
+        error.toStringDeep(),
+        'FlutterError\n'
+        '   Build scheduled during frame.\n'
+        '   While the widget tree was being built, laid out, and painted, a\n'
+        '   new frame was scheduled to rebuild the widget tree.\n'
+        '   This might be because setState() was called from a layout or\n'
+        '   paint callback. If a change is needed to the widget tree, it\n'
+        '   should be applied as the tree is being built. Scheduling a change\n'
+        '   for the subsequent frame instead results in an interface that\n'
+        '   lags behind by one frame. If this was done to make your build\n'
+        '   dependent on a size measured at layout time, consider using a\n'
+        '   LayoutBuilder, CustomSingleChildLayout, or\n'
+        '   CustomMultiChildLayout. If, on the other hand, the one frame\n'
+        '   delay is the desired effect, for example because this is an\n'
+        '   animation, consider scheduling the frame in a post-frame callback\n'
+        '   using SchedulerBinding.addPostFrameCallback or using an\n'
+        '   AnimationController to trigger the animation.\n',
+      );
+    }
+  });
 }
 
 class NullChildTest extends Widget {
@@ -681,3 +734,23 @@ class NullChildElement extends Element {
   @override
   void performRebuild() { }
 }
+
+
+class DirtyElementWithCustomBuildOwner extends Element {
+  DirtyElementWithCustomBuildOwner(BuildOwner buildOwner, Widget widget)
+    : _owner = buildOwner, super(widget);
+
+  final BuildOwner _owner;
+
+  @override
+  void forgetChild(Element child) {}
+
+  @override
+  void performRebuild() {}
+
+  @override
+  BuildOwner get owner => _owner;
+
+  @override
+  bool get dirty => true;
+}

packages/flutter/test/widgets/gesture_detector_test.dart

@@ -601,6 +601,104 @@ void main() {
         'excludeFromSemantics: true',
       ]);
     });
+
+    group('error control test', () {
+      test('constructor redundant pan and scale', () {
+        FlutterError error;
+        try {
+          GestureDetector(onScaleStart: (_) {}, onPanStart: (_) {},);
+        } on FlutterError catch (e) {
+          error = e;
+        } finally {
+          expect(error, isNotNull);
+          expect(
+            error.toStringDeep(),
+            'FlutterError\n'
+            '   Incorrect GestureDetector arguments.\n'
+            '   Having both a pan gesture recognizer and a scale gesture\n'
+            '   recognizer is redundant; scale is a superset of pan.\n'
+            '   Just use the scale gesture recognizer.\n',
+          );
+          expect(error.diagnostics.last.level, DiagnosticLevel.hint);
+          expect(
+            error.diagnostics.last.toStringDeep(),
+            equalsIgnoringHashCodes(
+              'Just use the scale gesture recognizer.\n',
+            )
+          );
+        }
+      });
+
+      test('constructur duplicate drag recognizer', () {
+        FlutterError error;
+        try {
+          GestureDetector(
+            onVerticalDragStart: (_) {},
+            onHorizontalDragStart: (_) {},
+            onPanStart: (_) {},
+          );
+        } on FlutterError catch (e) {
+          error = e;
+        } finally {
+          expect(error, isNotNull);
+          expect(
+            error.toStringDeep(),
+            'FlutterError\n'
+            '   Incorrect GestureDetector arguments.\n'
+            '   Simultaneously having a vertical drag gesture recognizer, a\n'
+            '   horizontal drag gesture recognizer, and a pan gesture recognizer\n'
+            '   will result in the pan gesture recognizer being ignored, since\n'
+            '   the other two will catch all drags.\n',
+          );
+        }
+      });
+
+      testWidgets('replaceGestureRecognizers not during layout', (WidgetTester tester) async {
+        final GlobalKey<RawGestureDetectorState> key = GlobalKey<RawGestureDetectorState>();
+        await tester.pumpWidget(
+          Directionality(
+            textDirection: TextDirection.ltr,
+            child: RawGestureDetector(
+              key: key,
+              child: Container(
+                child: const Text('Text'),
+              ),
+            ),
+          ),
+        );
+        FlutterError error;
+        try {
+          key.currentState.replaceGestureRecognizers(
+            <Type, GestureRecognizerFactory>{});
+        } on FlutterError catch (e) {
+          error = e;
+        } finally {
+          expect(error, isNotNull);
+          expect(error.diagnostics.last.level, DiagnosticLevel.hint);
+          expect(
+            error.diagnostics.last.toStringDeep(),
+            equalsIgnoringHashCodes(
+              'To set the gesture recognizers at other times, trigger a new\n'
+              'build using setState() and provide the new gesture recognizers as\n'
+              'constructor arguments to the corresponding RawGestureDetector or\n'
+              'GestureDetector object.\n'
+            ),
+          );
+          expect(
+            error.toStringDeep(),
+            'FlutterError\n'
+            '   Unexpected call to replaceGestureRecognizers() method of\n'
+            '   RawGestureDetectorState.\n'
+            '   The replaceGestureRecognizers() method can only be called during\n'
+            '   the layout phase.\n'
+            '   To set the gesture recognizers at other times, trigger a new\n'
+            '   build using setState() and provide the new gesture recognizers as\n'
+            '   constructor arguments to the corresponding RawGestureDetector or\n'
+            '   GestureDetector object.\n',
+          );
+        }
+      });
+    });
   });
 }
 

packages/flutter/test/widgets/heroes_test.dart

@@ -4,6 +4,7 @@
 
 import 'dart:ui' as ui;
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/cupertino.dart';
@@ -546,7 +547,31 @@ Future<void> main() async {
 
     await tester.tap(find.text('push'));
     await tester.pump();
-    expect(tester.takeException(), isFlutterError);
+    final dynamic exception = tester.takeException();
+    expect(exception, isFlutterError);
+    final FlutterError error = exception;
+    expect(error.diagnostics.length, 3);
+    final DiagnosticsNode last = error.diagnostics.last;
+    expect(last, isInstanceOf<DiagnosticsProperty<StatefulElement>>());
+    expect(
+      last.toStringDeep(),
+      equalsIgnoringHashCodes(
+        '# Here is the subtree for one of the offending heroes: Hero\n',
+      ),
+    );
+    expect(last.style, DiagnosticsTreeStyle.dense);
+    expect(
+      error.toStringDeep(),
+      equalsIgnoringHashCodes(
+        'FlutterError\n'
+        '   There are multiple heroes that share the same tag within a\n'
+        '   subtree.\n'
+        '   Within each subtree for which heroes are to be animated (i.e. a\n'
+        '   PageRoute subtree), each Hero must have a unique non-null tag.\n'
+        '   In this case, multiple heroes had the following tag: a\n'
+        '   ├# Here is the subtree for one of the offending heroes: Hero\n',
+      ),
+    );
   });
 
   testWidgets('Hero push transition interrupted by a pop', (WidgetTester tester) async {

packages/flutter/test/widgets/list_body_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/src/foundation/assertions.dart';
+import 'package:flutter/src/painting/basic_types.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
@@ -106,4 +108,90 @@ void main() {
       ],
     );
   });
+
+  testWidgets('Limited space along main axis error', (WidgetTester tester) async {
+    final FlutterExceptionHandler oldHandler = FlutterError.onError;
+    final List<FlutterErrorDetails> errors = <FlutterErrorDetails>[];
+    FlutterError.onError = (FlutterErrorDetails error) => errors.add(error);
+    try {
+      await tester.pumpWidget(
+        SizedBox(
+          width: 100,
+          height: 100,
+          child: Directionality(
+            textDirection: TextDirection.rtl,
+            child: ListBody(
+              mainAxis: Axis.horizontal,
+              children: children,
+            ),
+          ),
+        ),
+      );
+    } finally {
+      FlutterError.onError = oldHandler;
+    }
+    expect(errors, isNotEmpty);
+    expect(errors.first.exception, isFlutterError);
+    expect(errors.first.exception.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   RenderListBody must have unlimited space along its main axis.\n'
+      '   RenderListBody does not clip or resize its children, so it must\n'
+      '   be placed in a parent that does not constrain the main axis.\n'
+      '   You probably want to put the RenderListBody inside a\n'
+      '   RenderViewport with a matching main axis.\n'
+    ));
+  });
+
+  testWidgets('Nested ListBody unbounded cross axis error', (WidgetTester tester) async {
+    final FlutterExceptionHandler oldHandler = FlutterError.onError;
+    final List<FlutterErrorDetails> errors = <FlutterErrorDetails>[];
+    FlutterError.onError = (FlutterErrorDetails error) => errors.add(error);
+    try {
+      await tester.pumpWidget(
+        Flex(
+          textDirection: TextDirection.ltr,
+          direction: Axis.horizontal,
+          children: <Widget>[
+            Directionality(
+              textDirection: TextDirection.ltr,
+              child: ListBody(
+                mainAxis: Axis.horizontal,
+                children: <Widget>[
+                  Flex(
+                    textDirection: TextDirection.ltr,
+                    direction: Axis.vertical,
+                    children: <Widget>[
+                      Directionality(
+                        textDirection: TextDirection.ltr,
+                        child: ListBody(
+                          mainAxis: Axis.vertical,
+                          children: children,
+                        ),
+                      ),
+                    ],
+                  ),
+                ],
+              ),
+            ),
+          ],
+        ),
+      );
+    } finally {
+      FlutterError.onError = oldHandler;
+    }
+    expect(errors, isNotEmpty);
+    expect(errors.first.exception, isFlutterError);
+    expect(errors.first.exception.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   RenderListBody must have a bounded constraint for its cross axis.\n'
+      '   RenderListBody forces its children to expand to fit the\n'
+      '   RenderListBody\'s container, so it must be placed in a parent that\n'
+      '   constrains the cross axis to a finite dimension.\n'
+      '   If you are attempting to nest a RenderListBody with one direction\n'
+      '   inside one of another direction, you will want to wrap the inner\n'
+      '   one inside a box that fixes the dimension in that direction, for\n'
+      '   example, a RenderIntrinsicWidth or RenderIntrinsicHeight object.\n'
+      '   This is relatively expensive, however.\n'
+    ));
+  });
 }

packages/flutter/test/widgets/media_query_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:ui' show Brightness;
+import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
@@ -19,7 +20,27 @@ void main() {
       ),
     );
     expect(tested, isTrue);
-    expect(tester.takeException(), isFlutterError);
+    final dynamic exception = tester.takeException();
+    expect(exception, isNotNull);
+    expect(exception ,isFlutterError);
+    final FlutterError error = exception;
+    expect(error.diagnostics.length, 3);
+    expect(error.diagnostics.last, isInstanceOf<DiagnosticsProperty<Element>>());
+    expect(
+      error.toStringDeep(),
+      equalsIgnoringHashCodes(
+        'FlutterError\n'
+        '   MediaQuery.of() called with a context that does not contain a\n'
+        '   MediaQuery.\n'
+        '   No MediaQuery ancestor could be found starting from the context\n'
+        '   that was passed to MediaQuery.of(). This can happen because you\n'
+        '   do not have a WidgetsApp or MaterialApp widget (those widgets\n'
+        '   introduce a MediaQuery), or it can happen if the context you use\n'
+        '   comes from a widget above those widgets.\n'
+        '   The context used was:\n'
+        '     Builder\n',
+      ),
+    );
   });
 
   testWidgets('MediaQuery defaults to null', (WidgetTester tester) async {

packages/flutter/test/widgets/navigator_test.dart

@@ -1033,4 +1033,60 @@ void main() {
     expect(find.byKey(keyA), findsNothing);
     expect(find.byKey(keyAB), findsNothing);
   });
+
+  group('error control test', () {
+    testWidgets('onUnknownRoute null and onGenerateRoute returns null', (WidgetTester tester) async {
+      final GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
+      await tester.pumpWidget(Navigator(
+        key: navigatorKey,
+        onGenerateRoute: (_) => null,
+      ));
+      final dynamic exception = tester.takeException();
+      expect(exception, isNotNull);
+      expect(exception, isFlutterError);
+      final FlutterError error = exception;
+      expect(error, isNotNull);
+      expect(error.diagnostics.last, isInstanceOf<DiagnosticsProperty<NavigatorState>>());
+      expect(
+        error.toStringDeep(),
+        equalsIgnoringHashCodes(
+          'FlutterError\n'
+          '   If a Navigator has no onUnknownRoute, then its onGenerateRoute\n'
+          '   must never return null.\n'
+          '   When trying to build the route "/", onGenerateRoute returned\n'
+          '   null, but there was no onUnknownRoute callback specified.\n'
+          '   The Navigator was:\n'
+          '     NavigatorState#4d6bf(lifecycle state: created)\n',
+        ),
+      );
+    });
+
+    testWidgets('onUnknownRoute null and onGenerateRoute returns null', (WidgetTester tester) async {
+      final GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
+      await tester.pumpWidget(Navigator(
+        key: navigatorKey,
+        onGenerateRoute: (_) => null,
+        onUnknownRoute: (_) => null,
+      ));
+      final dynamic exception = tester.takeException();
+      expect(exception, isNotNull);
+      expect(exception, isFlutterError);
+      final FlutterError error = exception;
+      expect(error, isNotNull);
+      expect(error.diagnostics.last, isInstanceOf<DiagnosticsProperty<NavigatorState>>());
+      expect(
+        error.toStringDeep(),
+        equalsIgnoringHashCodes(
+          'FlutterError\n'
+          '   A Navigator\'s onUnknownRoute returned null.\n'
+          '   When trying to build the route "/", both onGenerateRoute and\n'
+          '   onUnknownRoute returned null. The onUnknownRoute callback should\n'
+          '   never return null.\n'
+          '   The Navigator was:\n'
+          '     NavigatorState#38036(lifecycle state: created)\n',
+        ),
+      );
+    });
+  });
+
 }

packages/flutter/test/widgets/overlay_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
@@ -612,4 +613,48 @@ void main() {
 
     expect(buildOrder, <int>[3, 4, 1, 2, 0]);
   });
+
+  testWidgets('OverlayState.of() called without Overlay being exist', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Builder(
+          builder: (BuildContext context) {
+            FlutterError error;
+            final Widget debugRequiredFor = Container();
+            try {
+              Overlay.of(context, debugRequiredFor: debugRequiredFor);
+            } on FlutterError catch (e) {
+              error = e;
+            } finally {
+              expect(error, isNotNull);
+              expect(error.diagnostics.length, 5);
+              expect(error.diagnostics[2].level, DiagnosticLevel.hint);
+              expect(error.diagnostics[2].toStringDeep(), equalsIgnoringHashCodes(
+                'The most common way to add an Overlay to an application is to\n'
+                'include a MaterialApp or Navigator widget in the runApp() call.\n',
+              ));
+              expect(error.diagnostics[3], isInstanceOf<DiagnosticsProperty<Widget>>());
+              expect(error.diagnostics[3].value, debugRequiredFor);
+              expect(error.diagnostics[4], isInstanceOf<DiagnosticsProperty<Element>>());
+              expect(error.toStringDeep(), equalsIgnoringHashCodes(
+                'FlutterError\n'
+                '   No Overlay widget found.\n'
+                '   Container widgets require an Overlay widget ancestor for correct\n'
+                '   operation.\n'
+                '   The most common way to add an Overlay to an application is to\n'
+                '   include a MaterialApp or Navigator widget in the runApp() call.\n'
+                '   The specific widget that failed to find an overlay was:\n'
+                '     Container\n'
+                '   The context from which that widget was searching for an overlay\n'
+                '   was:\n'
+                '     Builder\n',
+              ));
+            }
+            return Container();
+          }
+        ),
+      ),
+    );
+  });
 }

packages/flutter/test/widgets/scroll_physics_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -191,4 +193,51 @@ void main() {
       expect(smallListOverscrollApplied, lessThan(20.0));
     });
   });
+
+  test('ClampingScrollPhysics assertion test', () {
+    const ClampingScrollPhysics physics = ClampingScrollPhysics();
+    const double pixels = 500;
+    final ScrollMetrics position = FixedScrollMetrics(
+      pixels: pixels,
+      minScrollExtent: 0,
+      maxScrollExtent: 1000,
+      viewportDimension: 0,
+      axisDirection: AxisDirection.down,
+    );
+    expect(position.pixels, pixels);
+    FlutterError error;
+    try {
+      physics.applyBoundaryConditions(position, pixels);
+    } on FlutterError catch (e) {
+      error = e;
+    } finally {
+      expect(error, isNotNull);
+      expect(error.diagnostics.length, 4);
+      expect(error.diagnostics[2], isInstanceOf<DiagnosticsProperty<ScrollPhysics>>());
+      expect(error.diagnostics[2].style, DiagnosticsTreeStyle.errorProperty);
+      expect(error.diagnostics[2].value, physics);
+      expect(error.diagnostics[3], isInstanceOf<DiagnosticsProperty<ScrollMetrics>>());
+      expect(error.diagnostics[3].style, DiagnosticsTreeStyle.errorProperty);
+      expect(error.diagnostics[3].value, position);
+      // RegExp matcher is required here due to flutter web and flutter mobile generating
+      // slightly different floating point numbers
+      // in Flutter web 0.0 sometimes just appears as 0. or 0
+      expect(error.toStringDeep(), matches(RegExp(
+        r'''FlutterError
+   ClampingScrollPhysics\.applyBoundaryConditions\(\) was called
+   redundantly\.
+   The proposed new position\, 500(\.\d*)?, is exactly equal to the current
+   position of the given FixedScrollMetrics, 500(\.\d*)?\.
+   The applyBoundaryConditions method should only be called when the
+   value is going to actually change the pixels, otherwise it is
+   redundant\.
+   The physics object in question was\:
+     ClampingScrollPhysics
+   The position object in question was\:
+     FixedScrollMetrics\(500(\.\d*)?..\[0(\.\d*)?\]..500(\.\d*)?\)
+''',
+        multiLine: true,
+      )));
+    }
+  });
 }

packages/flutter_test/lib/src/widget_tester.dart

@@ -636,13 +636,16 @@ class WidgetTester extends WidgetController implements HitTestDispatcher, Ticker
     if (_tickers != null) {
       for (Ticker ticker in _tickers) {
         if (ticker.isActive) {
-          throw FlutterError(
-            'A Ticker was active $when.\n'
-            'All Tickers must be disposed. Tickers used by AnimationControllers '
-            'should be disposed by calling dispose() on the AnimationController itself. '
-            'Otherwise, the ticker will leak.\n'
-            'The offending ticker was: ${ticker.toString(debugIncludeStack: true)}'
-          );
+          throw FlutterError.fromParts(<DiagnosticsNode>[
+            ErrorSummary('A Ticker was active $when.'),
+            ErrorDescription('All Tickers must be disposed.'),
+            ErrorHint(
+              'Tickers used by AnimationControllers '
+              'should be disposed by calling dispose() on the AnimationController itself. '
+              'Otherwise, the ticker will leak.'
+            ),
+            ticker.describeForError('The offending ticker was')
+          ]);
         }
       }
     }
@@ -656,13 +659,20 @@ class WidgetTester extends WidgetController implements HitTestDispatcher, Ticker
   void _verifySemanticsHandlesWereDisposed() {
     assert(_lastRecordedSemanticsHandles != null);
     if (binding.pipelineOwner.debugOutstandingSemanticsHandles > _lastRecordedSemanticsHandles) {
-      throw FlutterError(
-        'A SemanticsHandle was active at the end of the test.\n'
-        'All SemanticsHandle instances must be disposed by calling dispose() on '
-        'the SemanticsHandle. If your test uses SemanticsTester, it is '
-        'sufficient to call dispose() on SemanticsTester. Otherwise, the '
-        'existing handle will leak into another test and alter its behavior.'
-      );
+      // TODO(jacobr): The hint for this one causes a change in line breaks but
+      // I think it is for the best.
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('A SemanticsHandle was active at the end of the test.'),
+        ErrorDescription(
+          'All SemanticsHandle instances must be disposed by calling dispose() on '
+          'the SemanticsHandle.'
+        ),
+        ErrorHint(
+          'If your test uses SemanticsTester, it is '
+          'sufficient to call dispose() on SemanticsTester. Otherwise, the '
+          'existing handle will leak into another test and alter its behavior.'
+        )
+      ]);
     }
     _lastRecordedSemanticsHandles = null;
   }

packages/flutter_test/test/widget_tester_test.dart

@@ -9,6 +9,7 @@ import 'dart:ui';
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/scheduler.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:test_api/test_api.dart' as test_package;
 import 'package:test_api/src/frontend/async_matcher.dart' show AsyncMatcher;
@@ -662,6 +663,41 @@ void main() {
       expect(find.text('Item 15', skipOffstage: true), findsOneWidget);
     });
   });
+
+  testWidgets('verifyTickersWereDisposed control test', (WidgetTester tester) async {
+      FlutterError error;
+      final Ticker ticker = tester.createTicker((Duration duration) {});
+      ticker.start();
+      try {
+        tester.verifyTickersWereDisposed('');
+      } on FlutterError catch (e) {
+        error = e;
+      } finally {
+        expect(error, isNotNull);
+        expect(error.diagnostics.length, 4);
+        expect(error.diagnostics[2].level, DiagnosticLevel.hint);
+        expect(
+          error.diagnostics[2].toStringDeep(),
+          'Tickers used by AnimationControllers should be disposed by\n'
+          'calling dispose() on the AnimationController itself. Otherwise,\n'
+          'the ticker will leak.\n',
+        );
+        expect(error.diagnostics.last, isInstanceOf<DiagnosticsProperty<Ticker>>());
+        expect(error.diagnostics.last.value, ticker);
+        expect(error.toStringDeep(), startsWith(
+          'FlutterError\n'
+          '   A Ticker was active .\n'
+          '   All Tickers must be disposed.\n'
+          '   Tickers used by AnimationControllers should be disposed by\n'
+          '   calling dispose() on the AnimationController itself. Otherwise,\n'
+          '   the ticker will leak.\n'
+          '   The offending ticker was:\n'
+          '     _TestTicker()\n',
+        ));
+      }
+      ticker.stop();
+  });
+
 }
 
 class FakeMatcher extends AsyncMatcher {
@@ -679,3 +715,28 @@ class FakeMatcher extends AsyncMatcher {
   @override
   Description describe(Description description) => description.add('--fake--');
 }
+
+class _SingleTickerTest extends StatefulWidget {
+  const _SingleTickerTest({Key key}) : super(key: key);
+
+  @override
+  _SingleTickerTestState createState() => _SingleTickerTestState();
+}
+
+class _SingleTickerTestState extends State<_SingleTickerTest> with SingleTickerProviderStateMixin {
+  AnimationController controller;
+
+  @override
+  void initState() {
+    super.initState();
+    controller = AnimationController(
+      vsync: this,
+      duration: const Duration(seconds: 100),
+    )  ;
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Container();
+  }
+}