Improve exceptions and asserts for rendering lib.

* Use actual exceptions rather than assertions containing code
  containing strings when trying to give messages to authors.
* Introduce RenderingError which is an AssertionError that takes a
  string argument, to support the above.
* Provide a BoxDimensions.hasBoundedWidth/hasBoundedHeight API.
* Document BoxDimensions.isNormalized.
* Provide more useful information when we assert isNormalized and find
  that it is false.
* When finding the size is infinite, crawl the tree to figure out which
  render box is likely responsible for the infinite constraints.
* Provide more information when size doesn't match the constraints.
* Provide more information when intrinsic dimension methods violate the
  constraints.
* Only spam a huge amount of information for the first exception from
  the rendering library. I've noticed a lot of people looking at the
  last exception printed rather than the first and that's very
  misleading -- after the rendering library hits an exception, all bets
  are off regarding what'll happen in the future. All kinds of asserts
  might fire.
* Improve docs around the debug methods and flags for the above.
* Make Block default to have no children. Previously, giving no children
  crashed with a confusing message about a null deref in an assert.

examples/rendering/lib/sector_layout.dart

@@ -79,16 +79,16 @@ abstract class RenderSector extends RenderObject {
   }
 
   SectorConstraints get constraints => super.constraints;
-  bool debugDoesMeetConstraints() {
+  void debugAssertDoesMeetConstraints() {
     assert(constraints != null);
     assert(deltaRadius != null);
     assert(deltaRadius < double.INFINITY);
     assert(deltaTheta != null);
     assert(deltaTheta < double.INFINITY);
-    return constraints.minDeltaRadius <= deltaRadius &&
-           deltaRadius <= math.max(constraints.minDeltaRadius, constraints.maxDeltaRadius) &&
-           constraints.minDeltaTheta <= deltaTheta &&
-           deltaTheta <= math.max(constraints.minDeltaTheta, constraints.maxDeltaTheta);
+    assert(constraints.minDeltaRadius <= deltaRadius);
+    assert(deltaRadius <= math.max(constraints.minDeltaRadius, constraints.maxDeltaRadius));
+    assert(constraints.minDeltaTheta <= deltaTheta);
+    assert(deltaTheta <= math.max(constraints.minDeltaTheta, constraints.maxDeltaTheta));
   }
   void performResize() {
     // default behaviour for subclasses that have sizedByParent = true

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

@@ -350,8 +350,9 @@ class RenderBlockViewport extends RenderBlockBase {
     double result;
     if (intrinsicCallback == null) {
       assert(() {
-        'RenderBlockViewport does not support returning intrinsic dimensions if the relevant callbacks have not been specified.';
-        return RenderObject.debugInDebugDoesMeetConstraints;
+        if (!RenderObject.debugCheckingIntrinsics)
+          throw new UnsupportedError('$runtimeType does not support returning intrinsic dimensions if the relevant callbacks have not been specified.');
+        return true;
       });
       return constrainer(0.0);
     }

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

@@ -98,7 +98,7 @@ class BoxConstraints extends Constraints {
   /// Returns new box constraints that are smaller by the given edge dimensions.
   BoxConstraints deflate(EdgeDims edges) {
     assert(edges != null);
-    assert(isNormalized);
+    assert(debugAssertIsNormalized);
     double horizontal = edges.left + edges.right;
     double vertical = edges.top + edges.bottom;
     double deflatedMinWidth = math.max(0.0, minWidth - horizontal);
@@ -113,7 +113,7 @@ class BoxConstraints extends Constraints {
 
   /// Returns new box constraints that remove the minimum width and height requirements.
   BoxConstraints loosen() {
-    assert(isNormalized);
+    assert(debugAssertIsNormalized);
     return new BoxConstraints(
       minWidth: 0.0,
       maxWidth: maxWidth,
@@ -154,14 +154,14 @@ class BoxConstraints extends Constraints {
   /// Returns the width that both satisfies the constraints and is as close as
   /// possible to the given width.
   double constrainWidth([double width = double.INFINITY]) {
-    assert(isNormalized);
+    assert(debugAssertIsNormalized);
     return width.clamp(minWidth, maxWidth);
   }
 
   /// Returns the height that both satisfies the constraints and is as close as
   /// possible to the given height.
   double constrainHeight([double height = double.INFINITY]) {
-    assert(isNormalized);
+    assert(debugAssertIsNormalized);
     return height.clamp(minHeight, maxHeight);
   }
 
@@ -192,9 +192,15 @@ class BoxConstraints extends Constraints {
   /// Whether there is exactly one size that satifies the constraints.
   bool get isTight => hasTightWidth && hasTightHeight;
 
+  /// Whether there is an upper bound on the maximum width.
+  bool get hasBoundedWidth => maxWidth < double.INFINITY;
+
+  /// Whether there is an upper bound on the maximum height.
+  bool get hasBoundedHeight => maxHeight < double.INFINITY;
+
   /// Whether the given size satisfies the constraints.
   bool isSatisfiedBy(Size size) {
-    assert(isNormalized);
+    assert(debugAssertIsNormalized);
     return (minWidth <= size.width) && (size.width <= maxWidth) &&
            (minHeight <= size.height) && (size.height <= maxHeight);
   }
@@ -245,8 +251,8 @@ class BoxConstraints extends Constraints {
       return b * t;
     if (b == null)
       return a * (1.0 - t);
-    assert(a.isNormalized);
-    assert(b.isNormalized);
+    assert(a.debugAssertIsNormalized);
+    assert(b.debugAssertIsNormalized);
     return new BoxConstraints(
       minWidth: ui.lerpDouble(a.minWidth, b.minWidth, t),
       maxWidth: ui.lerpDouble(a.maxWidth, b.maxWidth, t),
@@ -255,8 +261,34 @@ class BoxConstraints extends Constraints {
     );
   }
 
+  /// Returns whether the object's constraints are normalized.
+  /// Constraints are normalised if the minimums are less than or
+  /// equal to the corresponding maximums.
+  ///
+  /// For example, a BoxConstraints object with a minWidth of 100.0
+  /// and a maxWidth of 90.0 is not normalized.
+  ///
+  /// Most of the APIs on BoxConstraints expect the constraints to be
+  /// normalized and have undefined behavior when they are not. In
+  /// checked mode, many of these APIs will assert if the constraints
+  /// are not normalized.
   bool get isNormalized => minWidth <= maxWidth && minHeight <= maxHeight;
 
+  /// Same as [isNormalized] but, in checked mode, throws an exception
+  /// if isNormalized is false.
+  bool get debugAssertIsNormalized {
+    assert(() {
+      if (maxWidth < minWidth && maxHeight < minHeight)
+        throw new RenderingError('BoxConstraints has both width and height constraints non-normalized.\n$this');
+      if (maxWidth < minWidth)
+        throw new RenderingError('BoxConstraints has non-normalized width constraints.\n$this');
+      if (maxHeight < minHeight)
+        throw new RenderingError('BoxConstraints has non-normalized height constraints.\n$this');
+      return isNormalized;
+    });
+    return isNormalized;
+  }
+
   BoxConstraints normalize() {
     return new BoxConstraints(
       minWidth: minWidth,
@@ -267,13 +299,13 @@ class BoxConstraints extends Constraints {
   }
 
   bool operator ==(dynamic other) {
-    assert(isNormalized);
+    assert(debugAssertIsNormalized);
     if (identical(this, other))
       return true;
     if (other is! BoxConstraints)
       return false;
     final BoxConstraints typedOther = other;
-    assert(typedOther.isNormalized);
+    assert(typedOther.debugAssertIsNormalized);
     return minWidth == typedOther.minWidth &&
            maxWidth == typedOther.maxWidth &&
            minHeight == typedOther.minHeight &&
@@ -281,7 +313,7 @@ class BoxConstraints extends Constraints {
   }
 
   int get hashCode {
-    assert(isNormalized);
+    assert(debugAssertIsNormalized);
     return hashValues(minWidth, maxWidth, minHeight, maxHeight);
   }
 
@@ -362,7 +394,7 @@ abstract class RenderBox extends RenderObject {
   ///
   /// Override in subclasses that implement [performLayout].
   double getMinIntrinsicWidth(BoxConstraints constraints) {
-    assert(constraints.isNormalized);
+    assert(constraints.debugAssertIsNormalized);
     return constraints.constrainWidth(0.0);
   }
 
@@ -371,7 +403,7 @@ abstract class RenderBox extends RenderObject {
   ///
   /// Override in subclasses that implement [performLayout].
   double getMaxIntrinsicWidth(BoxConstraints constraints) {
-    assert(constraints.isNormalized);
+    assert(constraints.debugAssertIsNormalized);
     return constraints.constrainWidth(0.0);
   }
 
@@ -380,7 +412,7 @@ abstract class RenderBox extends RenderObject {
   ///
   /// Override in subclasses that implement [performLayout].
   double getMinIntrinsicHeight(BoxConstraints constraints) {
-    assert(constraints.isNormalized);
+    assert(constraints.debugAssertIsNormalized);
     return constraints.constrainHeight(0.0);
   }
 
@@ -393,7 +425,7 @@ abstract class RenderBox extends RenderObject {
   ///
   /// Override in subclasses that implement [performLayout].
   double getMaxIntrinsicHeight(BoxConstraints constraints) {
-    assert(constraints.isNormalized);
+    assert(constraints.debugAssertIsNormalized);
     return constraints.constrainHeight(0.0);
   }
 
@@ -447,7 +479,7 @@ abstract class RenderBox extends RenderObject {
       _size = new _DebugSize(_size, this, debugCanParentUseSize);
       return true;
     });
-    assert(debugDoesMeetConstraints());
+    assert(() { debugAssertDoesMeetConstraints(); return true; });
   }
 
   Rect get semanticBounds => Point.origin & size;
@@ -532,32 +564,91 @@ abstract class RenderBox extends RenderObject {
 
   /// The box constraints most recently received from the parent.
   BoxConstraints get constraints => super.constraints;
-  bool debugDoesMeetConstraints() {
-    assert(!RenderObject.debugInDebugDoesMeetConstraints);
-    RenderObject.debugInDebugDoesMeetConstraints = true;
+  void debugAssertDoesMeetConstraints() {
     assert(constraints != null);
-    // verify that the size is not infinite
     assert(_size != null);
-    assert(() {
-      'See https://flutter.io/layout/#unbounded-constraints';
-      return !_size.isInfinite;
-    });
+    // verify that the size is not infinite
+    if (_size.isInfinite) {
+      StringBuffer information = new StringBuffer();
+      if (!constraints.hasBoundedWidth) {
+        RenderBox node = this;
+        while (!node.constraints.hasBoundedWidth && node.parent is RenderBox)
+          node = node.parent;
+        information.writeln('The nearest ancestor providing an unbounded width constraint is:');
+        information.writeln('  $node');
+        List<String> settings = <String>[];
+        node.debugDescribeSettings(settings);
+        for (String line in settings)
+        information.writeln('  $line');
+      }
+      if (!constraints.hasBoundedHeight) {
+        RenderBox node = this;
+        while (!node.constraints.hasBoundedHeight && node.parent is RenderBox)
+          node = node.parent;
+        information.writeln('The nearest ancestor providing an unbounded height constraint is:');
+        information.writeln('  $node');
+        List<String> settings = <String>[];
+        node.debugDescribeSettings(settings);
+        for (String line in settings)
+        information.writeln('  $line');
+      }
+      throw new RenderingError(
+        '$runtimeType object was given an infinite size during layout.\n'
+        'This probably means that it is a render object that tries to be\n'
+        'as big as possible, but it was put inside another render object\n'
+        'that allows its children to pick their own size.\n'
+        '$information'
+        'See https://flutter.io/layout/ for more information.'
+      );
+    }
     // verify that the size is within the constraints
-    bool result = constraints.isSatisfiedBy(_size);
-    if (!result)
-      debugPrint("${this.runtimeType} does not meet its constraints. Constraints: $constraints, size: $_size");
+    if (!constraints.isSatisfiedBy(_size)) {
+      throw new RenderingError(
+        '$runtimeType does not meet its constraints.\n'
+        'Constraints: $constraints\n'
+        'Size: $_size\n'
+        'If you are not writing your own RenderBox subclass, then this is not\n'
+        'your fault. Contact support: https://github.com/flutter/flutter/issues/new'
+      );
+    }
     // verify that the intrinsics are also within the constraints
+    assert(!RenderObject.debugCheckingIntrinsics);
+    RenderObject.debugCheckingIntrinsics = true;
     double intrinsic;
+    StringBuffer failures = new StringBuffer();
+    int failureCount = 0;
     intrinsic = getMinIntrinsicWidth(constraints);
-    assert(intrinsic == constraints.constrainWidth(intrinsic));
+    if (intrinsic != constraints.constrainWidth(intrinsic)) {
+      failures.writeln(' * getMinIntrinsicWidth() -- returned: w=$intrinsic');
+      failureCount += 1;
+    }
     intrinsic = getMaxIntrinsicWidth(constraints);
-    assert(intrinsic == constraints.constrainWidth(intrinsic));
+    if (intrinsic != constraints.constrainWidth(intrinsic)) {
+      failures.writeln(' * getMaxIntrinsicWidth() -- returned: w=$intrinsic');
+      failureCount += 1;
+    }
     intrinsic = getMinIntrinsicHeight(constraints);
-    assert(intrinsic == constraints.constrainHeight(intrinsic));
+    if (intrinsic != constraints.constrainHeight(intrinsic)) {
+      failures.writeln(' * getMinIntrinsicHeight() -- returned: h=$intrinsic');
+      failureCount += 1;
+    }
     intrinsic = getMaxIntrinsicHeight(constraints);
-    assert(intrinsic == constraints.constrainHeight(intrinsic));
-    RenderObject.debugInDebugDoesMeetConstraints = false;
-    return result;
+    if (intrinsic != constraints.constrainHeight(intrinsic)) {
+      failures.writeln(' * getMaxIntrinsicHeight() -- returned: h=$intrinsic');
+      failureCount += 1;
+    }
+    RenderObject.debugCheckingIntrinsics = false;
+    if (failures.isNotEmpty) {
+      assert(failureCount > 0);
+      throw new RenderingError(
+        'The intrinsic dimension methods of the $runtimeType class returned values that violate the given constraints.\n'
+        'The constraints were: $constraints\n'
+        'The following method${failureCount > 1 ? "s" : ""} returned values outside of those constraints:\n'
+        '$failures'
+        'If you are not writing your own RenderBox subclass, then this is not\n'
+        'your fault. Contact support: https://github.com/flutter/flutter/issues/new'
+      );
+    }
   }
 
   void markNeedsLayout() {

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

@@ -724,11 +724,14 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   void visitChildren(RenderObjectVisitor visitor) { }
 
   dynamic debugOwner;
+  static int _debugPrintedExceptionCount = 0;
   void _debugReportException(String method, dynamic exception, StackTrace stack) {
     try {
       if (debugRenderingExceptionHandler != null) {
         debugRenderingExceptionHandler(this, method, exception, stack);
       } else {
+        _debugPrintedExceptionCount += 1;
+        if (_debugPrintedExceptionCount == 1) {
           debugPrint('-- EXCEPTION CAUGHT BY RENDERING LIBRARY -------------------------------');
           debugPrint('The following exception was raised during $method():');
           debugPrint('$exception');
@@ -754,22 +757,12 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
             debugPrint('This RenderObject has no descendants.');
           }
           descendants.forEach(debugPrint);
-        assert(() {
-          if (debugInDebugDoesMeetConstraints) {
-            debugPrint('This exception was thrown while debugDoesMeetConstraints() was running.');
-            debugPrint('debugDoesMeetConstraints() verifies that some invariants are not being');
-            debugPrint('violated. For example, it verifies that RenderBox objects are sized in');
-            debugPrint('a manner consistent with the constraints provided, and, in addition, that');
-            debugPrint('the getMinIntrinsicWidth(), getMaxIntrinsicWidth(), etc, functions all');
-            debugPrint('return consistent values within the same constraints.');
-            debugPrint('If you are not writing your own RenderObject subclass, then this is not');
-            debugPrint('your fault. Contact support: https://github.com/flutter/flutter/issues/new');
-          }
-          return true;
-        });
           debugPrint('Stack trace:');
           debugPrint('$stack');
           debugPrint('------------------------------------------------------------------------');
+        } else {
+          debugPrint('Another exception was raised: ${exception.toString().split("\n")[0]}');
+        }
       }
     } catch (exception) {
       debugPrint('(exception during exception handler: $exception)');
@@ -809,15 +802,23 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   Constraints _constraints;
   /// The layout constraints most recently supplied by the parent.
   Constraints get constraints => _constraints;
-  /// Override this function in a subclass to verify that your state matches the constraints object.
-  bool debugDoesMeetConstraints();
-  /// When true, debugDoesMeetConstraints() is currently executing.
-  ///
-  /// This should be set by implementations of debugDoesMeetConstraints() so that
-  /// tests can selectively ignore custom layout callbacks. It should not be set
-  /// outside of debugDoesMeetConstraints() implementations and should not be used
-  /// for purposes other than tests.
-  static bool debugInDebugDoesMeetConstraints = false;
+  /// Verify that the object's constraints are being met. Override
+  /// this function in a subclass to verify that your state matches
+  /// the constraints object. This function is only called in checked
+  /// mode. If the constraints are not met, it should assert or throw
+  /// an exception.
+  void debugAssertDoesMeetConstraints();
+
+  /// When true, debugAssertDoesMeetConstraints() is currently
+  /// executing asserts for verifying the consistent behaviour of
+  /// intrinsic dimensions methods.
+  ///
+  /// This should only be set by debugAssertDoesMeetConstraints()
+  /// implementations. It is used by tests to selectively ignore
+  /// custom layout callbacks. It should not be set outside of
+  /// debugAssertDoesMeetConstraints(), and should not be checked in
+  /// release mode (where it will always be false).
+  static bool debugCheckingIntrinsics = false;
   bool debugAncestorsAlreadyMarkedNeedsLayout() {
     if (_relayoutSubtreeRoot == null)
       return true; // we haven't yet done layout even once, so there's nothing for us to do
@@ -1022,7 +1023,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       assert(() { _debugDoingThisResize = true; return true; });
       try {
         performResize();
-        assert(debugDoesMeetConstraints());
+        assert(() { debugAssertDoesMeetConstraints(); return true; });
       } catch (e, stack) {
         _debugReportException('performResize', e, stack);
       }
@@ -1038,7 +1039,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     try {
       performLayout();
       markNeedsSemanticsUpdate();
-      assert(debugDoesMeetConstraints());
+      assert(() { debugAssertDoesMeetConstraints(); return true; });
     } catch (e, stack) {
       _debugReportException('performLayout', e, stack);
     }
@@ -2023,3 +2024,10 @@ abstract class ContainerRenderObjectMixin<ChildType extends RenderObject, Parent
     return result;
   }
 }
+
+/// Error thrown when the rendering library encounters a contract violation.
+class RenderingError extends AssertionError {
+  RenderingError(this.message);
+  final String message;
+  String toString() => message;
+}

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

@@ -78,7 +78,7 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
 
   // We never call layout() on this class, so this should never get
   // checked. (This class is laid out using scheduleInitialLayout().)
-  bool debugDoesMeetConstraints() { assert(false); return false; }
+  void debugAssertDoesMeetConstraints() { assert(false); }
 
   void performResize() {
     assert(false);

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

@@ -155,10 +155,14 @@ class _MixedViewportElement extends RenderObjectElement<MixedViewport> {
 
   double _noIntrinsicExtent(BoxConstraints constraints) {
     assert(() {
-      'MixedViewport does not support returning intrinsic dimensions. ' +
-      'Calculating the intrinsic dimensions would require walking the entire child list, ' +
-      'which defeats the entire point of having a lazily-built list of children.';
-      return RenderObject.debugInDebugDoesMeetConstraints;
+      if (!RenderObject.debugCheckingIntrinsics) {
+        throw new UnsupportedError(
+          'MixedViewport does not support returning intrinsic dimensions.\n'
+          'Calculating the intrinsic dimensions would require walking the entire child list,\n'
+          'which defeats the entire point of having a lazily-built list of children.'
+        );
+      }
+      return true;
     });
     return null;
   }

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

@@ -442,13 +442,14 @@ class ScrollableViewportState extends ScrollableState<ScrollableViewport> {
 class Block extends StatelessComponent {
   Block({
     Key key,
-    this.children,
+    this.children: const <Widget>[],
     this.padding,
     this.initialScrollOffset,
     this.scrollDirection: Axis.vertical,
     this.onScroll,
     this.scrollableKey
   }) : super(key: key) {
+    assert(children != null);
     assert(!children.any((Widget child) => child == null));
   }
 

packages/flutter/test/widget/custom_multi_child_layout_test.dart

@@ -11,7 +11,7 @@ class TestMultiChildLayoutDelegate extends MultiChildLayoutDelegate {
   BoxConstraints getSizeConstraints;
 
   Size getSize(BoxConstraints constraints) {
-    if (!RenderObject.debugInDebugDoesMeetConstraints)
+    if (!RenderObject.debugCheckingIntrinsics)
       getSizeConstraints = constraints;
     return new Size(200.0, 300.0);
   }
@@ -23,7 +23,7 @@ class TestMultiChildLayoutDelegate extends MultiChildLayoutDelegate {
   bool performLayoutIsChild;
 
   void performLayout(Size size, BoxConstraints constraints) {
-    assert(!RenderObject.debugInDebugDoesMeetConstraints);
+    assert(!RenderObject.debugCheckingIntrinsics);
     expect(() {
       performLayoutSize = size;
       performLayoutConstraints = constraints;
@@ -36,7 +36,7 @@ class TestMultiChildLayoutDelegate extends MultiChildLayoutDelegate {
   bool shouldRelayoutCalled = false;
   bool shouldRelayoutValue = false;
   bool shouldRelayout(_) {
-    assert(!RenderObject.debugInDebugDoesMeetConstraints);
+    assert(!RenderObject.debugCheckingIntrinsics);
     shouldRelayoutCalled = true;
     return shouldRelayoutValue;
   }

packages/flutter/test/widget/custom_one_child_layout_test.dart

@@ -14,13 +14,13 @@ class TestOneChildLayoutDelegate extends OneChildLayoutDelegate {
   Size childSizeFromGetPositionForChild;
 
   Size getSize(BoxConstraints constraints) {
-    if (!RenderObject.debugInDebugDoesMeetConstraints)
+    if (!RenderObject.debugCheckingIntrinsics)
       constraintsFromGetSize = constraints;
     return new Size(200.0, 300.0);
   }
 
   BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
-    assert(!RenderObject.debugInDebugDoesMeetConstraints);
+    assert(!RenderObject.debugCheckingIntrinsics);
     constraintsFromGetConstraintsForChild = constraints;
     return new BoxConstraints(
       minWidth: 100.0,
@@ -31,7 +31,7 @@ class TestOneChildLayoutDelegate extends OneChildLayoutDelegate {
   }
 
   Offset getPositionForChild(Size size, Size childSize) {
-    assert(!RenderObject.debugInDebugDoesMeetConstraints);
+    assert(!RenderObject.debugCheckingIntrinsics);
     sizeFromGetPositionForChild = size;
     childSizeFromGetPositionForChild = childSize;
     return Offset.zero;
@@ -40,7 +40,7 @@ class TestOneChildLayoutDelegate extends OneChildLayoutDelegate {
   bool shouldRelayoutCalled = false;
   bool shouldRelayoutValue = false;
   bool shouldRelayout(_) {
-    assert(!RenderObject.debugInDebugDoesMeetConstraints);
+    assert(!RenderObject.debugCheckingIntrinsics);
     shouldRelayoutCalled = true;
     return shouldRelayoutValue;
   }