Accessibility API for CustomPainter (#13313)

Summary:

- Add `key` field to `SemanticsNode`, while moving key into `foundation` library so it can be used by the render layer.
- Introduce `SemanticsProperties` and move many of the `Semantics` fields into it.
- Introduce `CustomPaintSemantics` - a `SemanticsNode` prototype created by `CustomPainter`.
- Introduce `semanticsBuilder` and `shouldRebuildSemantics` in `CustomerPainter`

**Breaking change**

The default `Semantics` constructor becomes non-const (due to https://github.com/dart-lang/sdk/issues/20962). However, a new `const Semantics.fromProperties` is added that still allowed creating constant `Semantics` widgets ([mailing list announcement](https://groups.google.com/forum/#!topic/flutter-dev/KQXBl2_1sws)).

Fixes https://github.com/flutter/flutter/issues/11791
Fixes https://github.com/flutter/flutter/issues/1666

packages/flutter/lib/foundation.dart

@@ -39,6 +39,7 @@ export 'src/foundation/change_notifier.dart';
 export 'src/foundation/collections.dart';
 export 'src/foundation/debug.dart';
 export 'src/foundation/diagnostics.dart';
+export 'src/foundation/key.dart';
 export 'src/foundation/licenses.dart';
 export 'src/foundation/node.dart';
 export 'src/foundation/observer_list.dart';

packages/flutter/lib/rendering.dart

@@ -35,6 +35,7 @@ export 'src/rendering/animated_size.dart';
 export 'src/rendering/binding.dart';
 export 'src/rendering/box.dart';
 export 'src/rendering/custom_layout.dart';
+export 'src/rendering/custom_paint.dart';
 export 'src/rendering/debug.dart';
 export 'src/rendering/debug_overflow_indicator.dart';
 export 'src/rendering/editable.dart';

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

+// Copyright 2017 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 'dart:ui' show hashValues;
+
+import 'package:meta/meta.dart';
+
+/// A [Key] is an identifier for [Widget]s, [Element]s and [SemanticsNode]s.
+///
+/// A new widget will only be used to update an existing element if its key is
+/// the same as the key of the current widget associated with the element.
+///
+/// Keys must be unique amongst the [Element]s with the same parent.
+///
+/// Subclasses of [Key] should either subclass [LocalKey] or [GlobalKey].
+///
+/// See also the discussion at [Widget.key].
+@immutable
+abstract class Key {
+  /// Construct a [ValueKey<String>] with the given [String].
+  ///
+  /// This is the simplest way to create keys.
+  const factory Key(String value) = ValueKey<String>;
+
+  /// Default constructor, used by subclasses.
+  ///
+  /// Useful so that subclasses can call us, because the [new Key] factory
+  /// constructor shadows the implicit constructor.
+  @protected
+  const Key.empty();
+}
+
+/// A key that is not a [GlobalKey].
+///
+/// Keys must be unique amongst the [Element]s with the same parent. By
+/// contrast, [GlobalKey]s must be unique across the entire app.
+///
+/// See also the discussion at [Widget.key].
+abstract class LocalKey extends Key {
+  /// Default constructor, used by subclasses.
+  const LocalKey() : super.empty();
+}
+
+/// A key that uses a value of a particular type to identify itself.
+///
+/// A [ValueKey<T>] is equal to another [ValueKey<T>] if, and only if, their
+/// values are [operator==].
+///
+/// This class can be subclassed to create value keys that will not be equal to
+/// other value keys that happen to use the same value. If the subclass is
+/// private, this results in a value key type that cannot collide with keys from
+/// other sources, which could be useful, for example, if the keys are being
+/// used as fallbacks in the same scope as keys supplied from another widget.
+///
+/// See also the discussion at [Widget.key].
+class ValueKey<T> extends LocalKey {
+  /// Creates a key that delegates its [operator==] to the given value.
+  const ValueKey(this.value);
+
+  /// The value to which this key delegates its [operator==]
+  final T value;
+
+  @override
+  bool operator ==(dynamic other) {
+    if (other.runtimeType != runtimeType)
+      return false;
+    final ValueKey<T> typedOther = other;
+    return value == typedOther.value;
+  }
+
+  @override
+  int get hashCode => hashValues(runtimeType, value);
+
+  @override
+  String toString() {
+    final String valueString = T == String ? '<\'$value\'>' : '<$value>';
+    // The crazy on the next line is a workaround for
+    // https://github.com/dart-lang/sdk/issues/28548
+    if (runtimeType == new _TypeLiteral<ValueKey<T>>().type)
+      return '[$valueString]';
+    return '[$T $valueString]';
+  }
+}
+
+class _TypeLiteral<T> { Type get type => T; }

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

@@ -6,6 +6,7 @@ import 'dart:async';
 import 'dart:math' as math;
 
 import 'package:flutter/foundation.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
 import 'theme.dart';
@@ -214,4 +215,10 @@ class _ScrollbarPainter extends ChangeNotifier implements CustomPainter {
 
   @override
   bool shouldRepaint(_ScrollbarPainter oldDelegate) => false;
+
+  @override
+  bool shouldRebuildSemantics(CustomPainter oldDelegate) => false;
+
+  @override
+  SemanticsBuilderCallback get semanticsBuilder => null;
 }

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

@@ -2362,7 +2362,7 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
   ///
   /// If [isSemanticBoundary] is true, this method is called with the `node`
   /// created for this [RenderObject], the `config` to be applied to that node
-  /// and the `children` [SemanticNode]s that decedents of this RenderObject
+  /// and the `children` [SemanticNode]s that descendants of this RenderObject
   /// have generated.
   ///
   /// By default, the method will annotate `node` with `config` and add the

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

@@ -17,6 +17,7 @@ export 'dart:ui' show hashValues, hashList;
 export 'package:flutter/foundation.dart' show FlutterError, debugPrint, debugPrintStack;
 export 'package:flutter/foundation.dart' show VoidCallback, ValueChanged, ValueGetter, ValueSetter;
 export 'package:flutter/foundation.dart' show DiagnosticLevel;
+export 'package:flutter/foundation.dart' show Key, LocalKey, ValueKey;
 export 'package:flutter/rendering.dart' show RenderObject, RenderBox, debugDumpRenderTree, debugDumpLayerTree;
 
 // Examples can assume:
@@ -30,84 +31,6 @@ export 'package:flutter/rendering.dart' show RenderObject, RenderBox, debugDumpR
 
 // KEYS
 
-/// A [Key] is an identifier for [Widget]s and [Element]s.
-///
-/// A new widget will only be used to update an existing element if its key is
-/// the same as the key of the current widget associated with the element.
-///
-/// Keys must be unique amongst the [Element]s with the same parent.
-///
-/// Subclasses of [Key] should either subclass [LocalKey] or [GlobalKey].
-///
-/// See also the discussion at [Widget.key].
-@immutable
-abstract class Key {
-  /// Construct a [ValueKey<String>] with the given [String].
-  ///
-  /// This is the simplest way to create keys.
-  const factory Key(String value) = ValueKey<String>;
-
-  /// Default constructor, used by subclasses.
-  ///
-  /// Useful so that subclasses can call us, because the Key() factory
-  /// constructor shadows the implicit constructor.
-  const Key._();
-}
-
-/// A key that is not a [GlobalKey].
-///
-/// Keys must be unique amongst the [Element]s with the same parent. By
-/// contrast, [GlobalKey]s must be unique across the entire app.
-///
-/// See also the discussion at [Widget.key].
-abstract class LocalKey extends Key {
-  /// Default constructor, used by subclasses.
-  const LocalKey() : super._();
-}
-
-/// A key that uses a value of a particular type to identify itself.
-///
-/// A [ValueKey<T>] is equal to another [ValueKey<T>] if, and only if, their
-/// values are [operator==].
-///
-/// This class can be subclassed to create value keys that will not be equal to
-/// other value keys that happen to use the same value. If the subclass is
-/// private, this results in a value key type that cannot collide with keys from
-/// other sources, which could be useful, for example, if the keys are being
-/// used as fallbacks in the same scope as keys supplied from another widget.
-///
-/// See also the discussion at [Widget.key].
-class ValueKey<T> extends LocalKey {
-  /// Creates a key that delegates its [operator==] to the given value.
-  const ValueKey(this.value);
-
-  /// The value to which this key delegates its [operator==]
-  final T value;
-
-  @override
-  bool operator ==(dynamic other) {
-    if (other.runtimeType != runtimeType)
-      return false;
-    final ValueKey<T> typedOther = other;
-    return value == typedOther.value;
-  }
-
-  @override
-  int get hashCode => hashValues(runtimeType, value);
-
-  @override
-  String toString() {
-    final String valueString = T == String ? '<\'$value\'>' : '<$value>';
-    // The crazy on the next line is a workaround for
-    // https://github.com/dart-lang/sdk/issues/28548
-    if (runtimeType == new _TypeLiteral<ValueKey<T>>().type)
-      return '[$valueString]';
-    return '[$T $valueString]';
-  }
-}
-
-class _TypeLiteral<T> { Type get type => T; }
-
 /// A key that is only equal to itself.
 class UniqueKey extends LocalKey {
   /// Creates a key that is equal only to itself.
@@ -183,7 +106,7 @@ abstract class GlobalKey<T extends State<StatefulWidget>> extends Key {
   ///
   /// Used by subclasses because the factory constructor shadows the implicit
   /// constructor.
-  const GlobalKey.constructor() : super._();
+  const GlobalKey.constructor() : super.empty();
 
   static final Map<GlobalKey, Element> _registry = <GlobalKey, Element>{};
   static final Set<GlobalKey> _removedKeys = new HashSet<GlobalKey>();
@@ -4287,9 +4210,10 @@ abstract class RenderObjectElement extends Element {
       return forgottenChildren != null && forgottenChildren.contains(child) ? null : child;
     }
 
-    // This attempts to diff the new child list (this.children) with
-    // the old child list (old.children), and update our renderObject
-    // accordingly.
+    // This attempts to diff the new child list (newWidgets) with
+    // the old child list (oldChildren), and produce a new list of elements to
+    // be the new list of child elements of this element. The called of this
+    // method is expected to update this render object accordingly.
 
     // The cases it tries to optimize for are:
     //  - the old list is empty
@@ -4305,13 +4229,13 @@ abstract class RenderObjectElement extends Element {
     // 2. Walk the lists from the bottom, without syncing nodes, until you no
     //    longer have matching nodes. We'll sync these nodes at the end. We
     //    don't sync them now because we want to sync all the nodes in order
-    //    from beginning ot end.
+    //    from beginning to end.
     // At this point we narrowed the old and new lists to the point
     // where the nodes no longer match.
     // 3. Walk the narrowed part of the old list to get the list of
     //    keys and sync null with non-keyed items.
     // 4. Walk the narrowed part of the new list forwards:
-    //     * Sync unkeyed items with null
+    //     * Sync non-keyed items with null
     //     * Sync keyed items with the source if it exists, else with null.
     // 5. Walk the bottom of the list again, syncing the nodes.
     // 6. Sync null with any items in the list of keys that are still
@@ -4378,7 +4302,7 @@ abstract class RenderObjectElement extends Element {
       if (haveOldChildren) {
         final Key key = newWidget.key;
         if (key != null) {
-          oldChild = oldKeyedChildren[newWidget.key];
+          oldChild = oldKeyedChildren[key];
           if (oldChild != null) {
             if (Widget.canUpdate(oldChild.widget, newWidget)) {
               // we found a match!
@@ -4400,7 +4324,7 @@ abstract class RenderObjectElement extends Element {
       newChildrenTop += 1;
     }
 
-    // We've scaned the whole list.
+    // We've scanned the whole list.
     assert(oldChildrenTop == oldChildrenBottom + 1);
     assert(newChildrenTop == newChildrenBottom + 1);
     assert(newWidgets.length - newChildrenTop == oldChildren.length - oldChildrenTop);
@@ -4423,7 +4347,7 @@ abstract class RenderObjectElement extends Element {
       oldChildrenTop += 1;
     }
 
-    // clean up any of the remaining middle nodes from the old list
+    // Clean up any of the remaining middle nodes from the old list.
     if (haveOldChildren && oldKeyedChildren.isNotEmpty) {
       for (Element oldChild in oldKeyedChildren.values) {
         if (forgottenChildren == null || !forgottenChildren.contains(oldChild))

packages/flutter/test/widgets/semantics_10_test.dart

@@ -47,10 +47,10 @@ Widget buildTestWidgets({bool excludeSemantics, String label, bool isSemanticsBo
           isSemanticBoundary: isSemanticsBoundary,
           child: new Column(
             children: <Widget>[
-              const Semantics(
+              new Semantics(
                 label: 'child1',
               ),
-              const Semantics(
+              new Semantics(
                 label: 'child2',
               ),
             ],

packages/flutter/test/widgets/semantics_11_test.dart

@@ -14,12 +14,12 @@ void main() {
     final SemanticsTester semantics = new SemanticsTester(tester);
 
     await tester.pumpWidget(
-      const Semantics(
+      new Semantics(
         container: true,
         onTap: dummyTapHandler,
-        child: const Semantics(
+        child: new Semantics(
           onTap: dummyTapHandler,
-          child: const Semantics(
+          child: new Semantics(
             onTap: dummyTapHandler,
             textDirection: TextDirection.ltr,
             label: 'foo',
@@ -54,12 +54,12 @@ void main() {
 
     // This should not throw an assert.
     await tester.pumpWidget(
-      const Semantics(
+      new Semantics(
         container: true,
         onTap: dummyTapHandler,
-        child: const Semantics(
+        child: new Semantics(
           onTap: dummyTapHandler,
-          child: const Semantics(
+          child: new Semantics(
             onTap: dummyTapHandler,
             textDirection: TextDirection.ltr,
             label: 'bar',  // <-- only change

packages/flutter/test/widgets/semantics_1_test.dart

@@ -50,7 +50,7 @@ void main() {
           children: <Widget>[
             new Container(
               height: 10.0,
-              child: const Semantics(
+              child: new Semantics(
                 label: 'child1',
                 textDirection: TextDirection.ltr,
                 selected: true,
@@ -58,9 +58,9 @@ void main() {
             ),
             new Container(
               height: 10.0,
-              child: const IgnorePointer(
+              child: new IgnorePointer(
                 ignoring: true,
-                child: const Semantics(
+                child: new Semantics(
                   label: 'child1',
                   textDirection: TextDirection.ltr,
                   selected: true,
@@ -92,7 +92,7 @@ void main() {
           children: <Widget>[
             new Container(
               height: 10.0,
-              child: const Semantics(
+              child: new Semantics(
                 label: 'child1',
                 textDirection: TextDirection.ltr,
                 selected: true,
@@ -100,9 +100,9 @@ void main() {
             ),
             new Container(
               height: 10.0,
-              child: const IgnorePointer(
+              child: new IgnorePointer(
                 ignoring: false,
-                child: const Semantics(
+                child: new Semantics(
                   label: 'child2',
                   textDirection: TextDirection.ltr,
                   selected: true,
@@ -146,7 +146,7 @@ void main() {
           children: <Widget>[
             new Container(
               height: 10.0,
-              child: const Semantics(
+              child: new Semantics(
                 label: 'child1',
                 textDirection: TextDirection.ltr,
                 selected: true,
@@ -154,9 +154,9 @@ void main() {
             ),
             new Container(
               height: 10.0,
-              child: const IgnorePointer(
+              child: new IgnorePointer(
                 ignoring: true,
-                child: const Semantics(
+                child: new Semantics(
                   label: 'child2',
                   textDirection: TextDirection.ltr,
                   selected: true,
@@ -188,7 +188,7 @@ void main() {
           children: <Widget>[
             new Container(
               height: 10.0,
-              child: const Semantics(
+              child: new Semantics(
                 label: 'child1',
                 textDirection: TextDirection.ltr,
                 selected: true,
@@ -196,9 +196,9 @@ void main() {
             ),
             new Container(
               height: 10.0,
-              child: const IgnorePointer(
+              child: new IgnorePointer(
                 ignoring: false,
-                child: const Semantics(
+                child: new Semantics(
                   label: 'child2',
                   textDirection: TextDirection.ltr,
                   selected: true,

packages/flutter/test/widgets/semantics_2_test.dart

@@ -28,7 +28,7 @@ void main() {
           children: <Widget>[
             new Container(
               height: 10.0,
-              child: const Semantics(
+              child: new Semantics(
                 label: 'child1',
                 textDirection: TextDirection.ltr,
                 selected: true,
@@ -36,9 +36,9 @@ void main() {
             ),
             new Container(
               height: 10.0,
-              child: const IgnorePointer(
+              child: new IgnorePointer(
                 ignoring: false,
-                child: const Semantics(
+                child: new Semantics(
                   label: 'child2',
                   textDirection: TextDirection.ltr,
                   selected: true,
@@ -82,7 +82,7 @@ void main() {
           children: <Widget>[
             new Container(
               height: 10.0,
-              child: const Semantics(
+              child: new Semantics(
                 label: 'child1',
                 textDirection: TextDirection.ltr,
                 selected: true,
@@ -90,9 +90,9 @@ void main() {
             ),
             new Container(
               height: 10.0,
-              child: const IgnorePointer(
+              child: new IgnorePointer(
                 ignoring: true,
-                child: const Semantics(
+                child: new Semantics(
                   label: 'child2',
                   textDirection: TextDirection.ltr,
                   selected: true,
@@ -124,7 +124,7 @@ void main() {
           children: <Widget>[
             new Container(
               height: 10.0,
-              child: const Semantics(
+              child: new Semantics(
                 label: 'child1',
                 textDirection: TextDirection.ltr,
                 selected: true,
@@ -132,9 +132,9 @@ void main() {
             ),
             new Container(
               height: 10.0,
-              child: const IgnorePointer(
+              child: new IgnorePointer(
                 ignoring: false,
-                child: const Semantics(
+                child: new Semantics(
                   label: 'child2',
                   textDirection: TextDirection.ltr,
                   selected: true,

packages/flutter/test/widgets/semantics_3_test.dart

@@ -23,7 +23,7 @@ void main() {
             label: 'test',
             textDirection: TextDirection.ltr,
             child: new Container(
-              child: const Semantics(
+              child: new Semantics(
                 checked: true
               ),
             ),
@@ -50,7 +50,7 @@ void main() {
       new Semantics(
         container: true,
         child: new Container(
-          child: const Semantics(
+          child: new Semantics(
              checked: true,
           ),
         ),
@@ -74,7 +74,7 @@ void main() {
       new Semantics(
         container: true,
         child: new Container(
-          child: const Semantics(
+          child: new Semantics(
             label: 'test',
             textDirection: TextDirection.ltr,
           ),
@@ -100,9 +100,9 @@ void main() {
       new Semantics(
         container: true,
         child: new Container(
-          child: const Semantics(
+          child: new Semantics(
             checked: true,
-            child: const Semantics(
+            child: new Semantics(
               label: 'test',
               textDirection: TextDirection.ltr,
             ),
@@ -134,9 +134,9 @@ void main() {
       new Semantics(
         container: true,
         child: new Container(
-          child: const Semantics(
+          child: new Semantics(
             checked: true,
-            child: const Semantics(
+            child: new Semantics(
               label: 'test',
               textDirection: TextDirection.ltr,
             ),

packages/flutter/test/widgets/semantics_4_test.dart

@@ -26,7 +26,7 @@ void main() {
       child: new Stack(
         fit: StackFit.expand,
         children: <Widget>[
-          const Semantics(
+          new Semantics(
             container: true,
             label: 'L1',
           ),
@@ -36,10 +36,10 @@ void main() {
             child: new Stack(
               fit: StackFit.expand,
               children: <Widget>[
-                const Semantics(
+                new Semantics(
                   checked: true,
                 ),
-                const Semantics(
+                new Semantics(
                   checked: false,
                 ),
               ],
@@ -88,7 +88,7 @@ void main() {
       child: new Stack(
         fit: StackFit.expand,
         children: <Widget>[
-          const Semantics(
+          new Semantics(
             label: 'L1',
             container: true,
           ),
@@ -98,10 +98,10 @@ void main() {
             child: new Stack(
               fit: StackFit.expand,
               children: <Widget>[
-                const Semantics(
+                new Semantics(
                   checked: true,
                 ),
-                const Semantics(),
+                new Semantics(),
               ],
             ),
           ),
@@ -136,17 +136,17 @@ void main() {
       child: new Stack(
         fit: StackFit.expand,
         children: <Widget>[
-          const Semantics(),
+          new Semantics(),
           new Semantics(
             label: 'L2',
             container: true,
             child: new Stack(
               fit: StackFit.expand,
               children: <Widget>[
-                const Semantics(
+                new Semantics(
                   checked: true,
                 ),
-                const Semantics(),
+                new Semantics(),
               ],
             ),
           ),

packages/flutter/test/widgets/semantics_5_test.dart

@@ -17,14 +17,14 @@ void main() {
         textDirection: TextDirection.ltr,
         fit: StackFit.expand,
         children: <Widget>[
-          const Semantics(
+          new Semantics(
             // this tests that empty nodes disappear
           ),
-          const Semantics(
+          new Semantics(
             // this tests whether you can have a container with no other semantics
             container: true,
           ),
-          const Semantics(
+          new Semantics(
             label: 'label', // (force a fork)
             textDirection: TextDirection.ltr,
           ),

packages/flutter/test/widgets/semantics_7_test.dart

@@ -37,7 +37,7 @@ void main() {
               child: new Stack(
                 fit: StackFit.expand,
                 children: <Widget>[
-                  const Semantics(
+                  new Semantics(
                     checked: true,
                   ),
                   new Semantics(
@@ -93,7 +93,7 @@ void main() {
               child: new Stack(
                 fit: StackFit.expand,
                 children: <Widget>[
-                  const Semantics(
+                  new Semantics(
                     checked: true,
                   ),
                   new Semantics(

packages/flutter/test/widgets/semantics_8_test.dart

@@ -23,10 +23,10 @@ void main() {
             child: new Stack(
               textDirection: TextDirection.ltr,
               children: <Widget>[
-                const Semantics(
+                new Semantics(
                   checked: true,
                 ),
-                const Semantics(
+                new Semantics(
                   label: 'label',
                   textDirection: TextDirection.ltr,
                 )
@@ -61,11 +61,11 @@ void main() {
             child: new Stack(
               textDirection: TextDirection.ltr,
               children: <Widget>[
-                const Semantics(
+                new Semantics(
                   label: 'label',
                   textDirection: TextDirection.ltr,
                 ),
-                const Semantics(
+                new Semantics(
                   checked: true
                 )
               ]

packages/flutter/test/widgets/semantics_debugger_test.dart

@@ -16,11 +16,11 @@ void main() {
         textDirection: TextDirection.ltr,
         child: new Stack(
           children: <Widget>[
-            const Semantics(),
-            const Semantics(
+            new Semantics(),
+            new Semantics(
               container: true,
             ),
-            const Semantics(
+            new Semantics(
               label: 'label',
               textDirection: TextDirection.ltr,
             ),
@@ -35,11 +35,11 @@ void main() {
         child: new SemanticsDebugger(
           child: new Stack(
             children: <Widget>[
-              const Semantics(),
-              const Semantics(
+              new Semantics(),
+              new Semantics(
                 container: true,
               ),
-              const Semantics(
+              new Semantics(
                 label: 'label',
                 textDirection: TextDirection.ltr,
               ),
@@ -62,14 +62,14 @@ void main() {
         child: new SemanticsDebugger(
           child: new Stack(
             children: <Widget>[
-              const Semantics(label: 'label1', textDirection: TextDirection.ltr),
+              new Semantics(label: 'label1', textDirection: TextDirection.ltr),
               new Positioned(
                 key: key,
                 left: 0.0,
                 top: 0.0,
                 width: 100.0,
                 height: 100.0,
-                child: const Semantics(label: 'label2', textDirection: TextDirection.ltr),
+                child: new Semantics(label: 'label2', textDirection: TextDirection.ltr),
               ),
             ],
           ),
@@ -83,7 +83,7 @@ void main() {
         child: new SemanticsDebugger(
           child: new Stack(
             children: <Widget>[
-              const Semantics(label: 'label1', textDirection: TextDirection.ltr),
+              new Semantics(label: 'label1', textDirection: TextDirection.ltr),
               new Semantics(
                 container: true,
                 child: new Stack(
@@ -94,9 +94,9 @@ void main() {
                       top: 0.0,
                       width: 100.0,
                       height: 100.0,
-                      child: const Semantics(label: 'label2', textDirection: TextDirection.ltr),
+                      child: new Semantics(label: 'label2', textDirection: TextDirection.ltr),
                     ),
-                    const Semantics(label: 'label3', textDirection: TextDirection.ltr),
+                    new Semantics(label: 'label3', textDirection: TextDirection.ltr),
                   ],
                 ),
               ),
@@ -112,7 +112,7 @@ void main() {
         child: new SemanticsDebugger(
           child: new Stack(
             children: <Widget>[
-              const Semantics(label: 'label1', textDirection: TextDirection.ltr),
+              new Semantics(label: 'label1', textDirection: TextDirection.ltr),
               new Semantics(
                 container: true,
                 child: new Stack(
@@ -123,9 +123,9 @@ void main() {
                         top: 0.0,
                         width: 100.0,
                         height: 100.0,
-                        child: const Semantics(label: 'label2', textDirection: TextDirection.ltr)),
-                    const Semantics(label: 'label3', textDirection: TextDirection.ltr),
-                    const Semantics(label: 'label4', textDirection: TextDirection.ltr),
+                        child: new Semantics(label: 'label2', textDirection: TextDirection.ltr)),
+                    new Semantics(label: 'label3', textDirection: TextDirection.ltr),
+                    new Semantics(label: 'label4', textDirection: TextDirection.ltr),
                   ],
                 ),
               ),

packages/flutter/test/widgets/semantics_test.dart

@@ -262,10 +262,10 @@ void main() {
           container: true,
           child: new Column(
             children: <Widget>[
-              const Semantics(
+              new Semantics(
                 hint: 'hint one',
               ),
-              const Semantics(
+              new Semantics(
                 hint: 'hint two',
               )
 
@@ -347,10 +347,10 @@ void main() {
           container: true,
           child: new Column(
             children: <Widget>[
-              const Semantics(
+              new Semantics(
                 hint: 'hint',
               ),
-              const Semantics(
+              new Semantics(
                 value: 'value',
               ),
             ],

packages/flutter/test/widgets/semantics_tester.dart

@@ -196,7 +196,7 @@ class TestSemantics {
     final SemanticsData nodeData = node.getSemanticsData();
 
     bool fail(String message) {
-      matchState[TestSemantics] = '$message\n$_matcherHelp';
+      matchState[TestSemantics] = '$message';
       return false;
     }
 
@@ -246,8 +246,29 @@ class TestSemantics {
   }
 
   @override
-  String toString() {
-    return 'node $id, flags=$flags, actions=$actions, label="$label", textDirection=$textDirection, rect=$rect, transform=$transform, ${children.length} child${ children.length == 1 ? "" : "ren" }';
+  String toString([int indentAmount = 0]) {
+    final String indent = '  ' * indentAmount;
+    final StringBuffer buf = new StringBuffer();
+    buf.writeln('$indent$runtimeType {');
+    if (id != null)
+      buf.writeln('$indent  id: $id');
+    buf.writeln('$indent  flags: $flags');
+    buf.writeln('$indent  actions: $actions');
+    if (label != null)
+      buf.writeln('$indent  label: "$label"');
+    if (textDirection != null)
+      buf.writeln('$indent  textDirection: $textDirection');
+    if (rect != null)
+      buf.writeln('$indent  rect: $rect');
+    if (transform != null)
+      buf.writeln('$indent  transform:\n${transform.toString().trim().split('\n').map((String line) => '$indent    $line').join('\n')}');
+    buf.writeln('$indent  children: [');
+    for (TestSemantics child in children) {
+      buf.writeln(child.toString(indentAmount + 2));
+    }
+    buf.writeln('$indent  ]');
+    buf.write('$indent}');
+    return buf.toString();
   }
 }
 
@@ -295,12 +316,17 @@ class _HasSemantics extends Matcher {
 
   @override
   Description describe(Description description) {
-    return description.add('semantics node matching: $_semantics');
+    return description.add('semantics node matching:\n$_semantics');
   }
 
   @override
   Description describeMismatch(dynamic item, Description mismatchDescription, Map<dynamic, dynamic> matchState, bool verbose) {
-    return mismatchDescription.add(matchState[TestSemantics]);
+    return mismatchDescription
+        .add('${matchState[TestSemantics]}\n')
+        .add(
+          'Current SemanticsNode tree:\n'
+        )
+        .add(RendererBinding.instance?.renderView?.debugSemantics?.toStringDeep(childOrder: DebugSemanticsDumpOrder.inverseHitTest));
   }
 }