Provide hooks for when exceptions are thrown.

This might be helpful for #1219.

Also, remove inDebugMode since it's redundant with just using asserts,
which compile entirely out in release mode.

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

@@ -444,28 +444,36 @@ abstract class RenderBox extends RenderObject {
   /// of those functins, call [markNeedsLayout] instead to schedule a layout of
   /// the box.
   Size get size {
-    if (_size is _DebugSize) {
-      final _DebugSize _size = this._size; // TODO(ianh): Remove this once the analyzer is cleverer
-      assert(_size._owner == this);
-      if (RenderObject.debugActiveLayout != null) {
-        // we are always allowed to access our own size (for print debugging and asserts if nothing else)
-        // other than us, the only object that's allowed to read our size is our parent, if they're said they will
-        // if you hit this assert trying to access a child's size, pass parentUsesSize: true in layout()
-        assert(debugDoingThisResize || debugDoingThisLayout ||
-               (RenderObject.debugActiveLayout == parent && _size._canBeUsedByParent));
+    assert(() {
+      if (_size is _DebugSize) {
+        final _DebugSize _size = this._size; // TODO(ianh): Remove this once the analyzer is cleverer
+        assert(_size._owner == this);
+        if (RenderObject.debugActiveLayout != null) {
+          // we are always allowed to access our own size (for print debugging and asserts if nothing else)
+          // other than us, the only object that's allowed to read our size is our parent, if they're said they will
+          // if you hit this assert trying to access a child's size, pass parentUsesSize: true in layout()
+          assert(debugDoingThisResize || debugDoingThisLayout ||
+                 (RenderObject.debugActiveLayout == parent && _size._canBeUsedByParent));
+        }
+        assert(_size == this._size); // TODO(ianh): Remove this once the analyzer is cleverer
       }
-      assert(_size == this._size); // TODO(ianh): Remove this once the analyzer is cleverer
-    }
+      return true;
+    });
     return _size;
   }
   void set size(Size value) {
     assert((sizedByParent && debugDoingThisResize) ||
            (!sizedByParent && debugDoingThisLayout));
-    if (value is _DebugSize) {
-      assert(value._canBeUsedByParent);
-      assert(value._owner.parent == this);
-    }
-    _size = inDebugBuild ? new _DebugSize(value, this, debugCanParentUseSize) : value;
+    assert(() {
+      if (value is _DebugSize)
+        return value._canBeUsedByParent && value._owner.parent == this;
+      return true;
+    });
+    _size = value;
+    assert(() {
+      _size = new _DebugSize(_size, this, debugCanParentUseSize);
+      return true;
+    });
     assert(debugDoesMeetConstraints());
   }
 

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

@@ -4,19 +4,6 @@
 
 import 'dart:sky' as sky;
 
-/// Indicates whether we're running with asserts enabled.
-final bool inDebugBuild = _initInDebugBuild();
-
-bool _initInDebugBuild() {
-  bool _inDebug = false;
-  bool setAssert() {
-    _inDebug = true;
-    return true;
-  }
-  assert(setAssert());
-  return _inDebug;
-}
-
 /// Causes each RenderBox to paint a box around its bounds.
 bool debugPaintSizeEnabled = false;
 
@@ -43,6 +30,3 @@ bool debugPaintBoundsEnabled = false;
 
 /// The color to use when painting RenderError boxes in checked mode.
 sky.Color debugErrorBoxColor = const sky.Color(0xFFFF0000);
-
-/// How many lines of debugging output to include when an exception is reported.
-int debugRenderObjectDumpMaxLength = 10;

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

@@ -368,6 +368,17 @@ typedef void RenderObjectVisitor(RenderObject child);
 typedef void LayoutCallback(Constraints constraints);
 typedef double ExtentCallback(Constraints constraints);
 
+typedef void RenderingExceptionHandler(RenderObject source, String method, dynamic exception, StackTrace stack);
+/// This callback is invoked whenever an exception is caught by the rendering
+/// system. The 'source' argument is the [RenderObject] object that caught the
+/// exception. The 'method' argument is the method in which the exception
+/// occurred; it will be one of 'performResize', 'performLayout, or 'paint'. The
+/// 'exception' argument contains the object that was thrown, and the 'stack'
+/// argument contains the stack trace. The callback is invoked after the
+/// information is printed to the console, and could be used to print additional
+/// information, such as from [debugDumpRenderTree()].
+RenderingExceptionHandler debugRenderingExceptionHandler;
+
 /// An object in the render tree
 ///
 /// Render objects have a reference to their parent but do not commit to a model
@@ -445,10 +456,11 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     print('$exception');
     print('Stack trace:');
     print('$stack');
-    'The following RenderObject was being processed when the exception was fired (limited to $debugRenderObjectDumpMaxLength lines):\n${this}'
-      .split('\n').take(debugRenderObjectDumpMaxLength+1).forEach(print);
+    print('The following RenderObject was being processed when the exception was fired:\n${this}');
     if (debugExceptionContext != '')
-      'The RenderObject had the following exception context:\n${debugExceptionContext}'.split('\n').forEach(print);
+      'That RenderObject had the following exception context:\n${debugExceptionContext}'.split('\n').forEach(print);
+    if (debugRenderingExceptionHandler != null)
+      debugRenderingExceptionHandler(this, method, exception, stack);
   }
 
   static bool _debugDoingLayout = false;

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

@@ -1655,6 +1655,16 @@ class ErrorWidget extends LeafRenderObjectWrapper {
   RenderBox createNode() => new RenderErrorBox();
 }
 
+typedef void WidgetsExceptionHandler(String context, dynamic exception, StackTrace stack);
+/// This callback is invoked whenever an exception is caught by the widget
+/// system. The 'context' argument is a description of what was happening when
+/// the exception occurred, and may include additional details such as
+/// descriptions of the objects involved. The 'exception' argument contains the
+/// object that was thrown, and the 'stack' argument contains the stack trace.
+/// The callback is invoked after the information is printed to the console, and
+/// could be used to print additional information, such as from
+/// [debugDumpApp()].
+WidgetsExceptionHandler debugWidgetsExceptionHandler;
 void _debugReportException(String context, dynamic exception, StackTrace stack) {
   print('------------------------------------------------------------------------');
   'Exception caught while $context'.split('\n').forEach(print);
@@ -1663,7 +1673,7 @@ void _debugReportException(String context, dynamic exception, StackTrace stack)
   '$stack'.split('\n').forEach(print);
   print('Build stack:');
   Component._debugComponentBuildTree.forEach((Component component) { print('  $component'); });
-  print('Current application widget tree:');
-  debugDumpApp();
+  if (debugWidgetsExceptionHandler != null)
+    debugWidgetsExceptionHandler(context, exception, stack);
   print('------------------------------------------------------------------------');
 }