Changing themes caused crash

The root cause was that we crawled the tree to mark anyone who depended
on the updated theme dirty _after_ we crawled it to rebuild it. Thus, if
anyone was already marked dirty when the process started, then got
marked clean by the first (rebuild) walk, then got marked dirty again by
the notification, they'd be clean when they got the notification,
despite already being in the dirty list, which would cause an assertion.

Also IconTheme didn't have an operator==, so it was independently too
aggressive about updates.

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

@@ -7,6 +7,8 @@ import 'dart:collection';
 
 import 'package:sky/rendering.dart';
 
+// KEYS
+
 /// A Key is an identifier for [Widget]s and [Element]s. A new Widget will only
 /// be used to reconfigure an existing Element if its Key is the same as its
 /// original Widget's Key.
@@ -171,6 +173,8 @@ class GlobalObjectKey extends GlobalKey {
 }
 
 
+// WIDGETS
+
 /// A Widget object describes the configuration for an [Element].
 /// Widget subclasses should be immutable with const constructors.
 /// Widgets form a tree that is then inflated into an Element tree.
@@ -193,6 +197,8 @@ abstract class Widget {
   void debugFillDescription(List<String> description) { }
 }
 
+// TODO(ianh): move the next four classes to below InheritedWidget
+
 /// RenderObjectWidgets provide the configuration for [RenderObjectElement]s,
 /// which wrap [RenderObject]s, which provide the actual rendering of the
 /// application.
@@ -382,16 +388,14 @@ abstract class State<T extends StatefulComponent> {
   }
 }
 
-abstract class ProxyWidget extends StatelessComponent {
+abstract class ProxyComponent extends Widget {
-  const ProxyWidget({ Key key, Widget this.child }) : super(key: key);
+  const ProxyComponent({ Key key, this.child }) : super(key: key);
 
   final Widget child;
-
-  Widget build(BuildContext context) => child;
 }
 
-abstract class ParentDataWidget extends ProxyWidget {
+abstract class ParentDataWidget extends ProxyComponent {
-  ParentDataWidget({ Key key, Widget child })
+  const ParentDataWidget({ Key key, Widget child })
     : super(key: key, child: child);
 
   ParentDataElement createElement() => new ParentDataElement(this);
@@ -405,7 +409,7 @@ abstract class ParentDataWidget extends ProxyWidget {
   void applyParentData(RenderObject renderObject);
 }
 
-abstract class InheritedWidget extends ProxyWidget {
+abstract class InheritedWidget extends ProxyComponent {
   const InheritedWidget({ Key key, Widget child })
     : super(key: key, child: child);
 
@@ -414,6 +418,9 @@ abstract class InheritedWidget extends ProxyWidget {
   bool updateShouldNotify(InheritedWidget oldWidget);
 }
 
+
+// ELEMENTS
+
 bool _canUpdate(Widget oldWidget, Widget newWidget) {
   return oldWidget.runtimeType == newWidget.runtimeType &&
          oldWidget.key == newWidget.key;
@@ -637,6 +644,7 @@ abstract class Element<T extends Widget> implements BuildContext {
     assert(_debugLifecycleState == _ElementLifecycle.active);
     assert(widget != null);
     assert(newWidget != null);
+    assert(newWidget != widget);
     assert(depth != null);
     assert(_active);
     assert(_canUpdate(widget, newWidget));
@@ -970,8 +978,8 @@ abstract class BuildableElement<T extends Widget> extends Element<T> {
 
 typedef Widget WidgetBuilder(BuildContext context);
 
-/// Base class for the instantiation of StatelessComponent and StatefulComponent
+/// Base class for the instantiation of StatelessComponent, StatefulComponent,
-/// widgets.
+/// and ProxyComponent widgets.
 abstract class ComponentElement<T extends Widget> extends BuildableElement<T> {
   ComponentElement(T widget) : super(widget);
 
@@ -1114,7 +1122,26 @@ class StatefulComponentElement<T extends StatefulComponent, U extends State<T>>
   }
 }
 
-class ParentDataElement extends StatelessComponentElement<ParentDataWidget> {
+abstract class ProxyElement<T extends ProxyComponent> extends ComponentElement<T> {
+  ProxyElement(T widget) : super(widget) {
+    _builder = (BuildContext context) => this.widget.child;
+  }
+
+  void update(T newWidget) {
+    T oldWidget = widget;
+    assert(widget != null);
+    assert(widget != newWidget);
+    super.update(newWidget);
+    assert(widget == newWidget);
+    notifyDescendants(oldWidget);
+    _dirty = true;
+    rebuild();
+  }
+
+  void notifyDescendants(T oldWidget);
+}
+
+class ParentDataElement extends ProxyElement<ParentDataWidget> {
   ParentDataElement(ParentDataWidget widget) : super(widget);
 
   void mount(Element parent, dynamic slot) {
@@ -1134,15 +1161,7 @@ class ParentDataElement extends StatelessComponentElement<ParentDataWidget> {
     super.mount(parent, slot);
   }
 
-  void update(ParentDataWidget newWidget) {
+  void notifyDescendants(ParentDataWidget oldWidget) {
-    ParentDataWidget oldWidget = widget;
-    super.update(newWidget);
-    assert(widget == newWidget);
-    if (widget != oldWidget)
-      _notifyDescendants();
-  }
-
-  void _notifyDescendants() {
     void notifyChildren(Element child) {
       if (child is RenderObjectElement)
         child.updateParentData(widget);
@@ -1153,20 +1172,14 @@ class ParentDataElement extends StatelessComponentElement<ParentDataWidget> {
   }
 }
 
-class InheritedElement extends StatelessComponentElement<InheritedWidget> {
+
+
+class InheritedElement extends ProxyElement<InheritedWidget> {
   InheritedElement(InheritedWidget widget) : super(widget);
 
-  void update(StatelessComponent newWidget) {
+  void notifyDescendants(InheritedWidget oldWidget) {
-    InheritedWidget oldWidget = widget;
+    if (!widget.updateShouldNotify(oldWidget))
-    super.update(newWidget);
+      return;
-    assert(widget == newWidget);
-    if (widget.updateShouldNotify(oldWidget)) {
-      assert(widget != oldWidget);
-      _notifyDescendants();
-    }
-  }
-
-  void _notifyDescendants() {
     final Type ourRuntimeType = widget.runtimeType;
     void notifyChildren(Element child) {
       if (child._dependencies != null &&

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

@@ -14,6 +14,9 @@ enum IconThemeColor { white, black }
 class IconThemeData {
   const IconThemeData({ this.color });
   final IconThemeColor color;
+
+  bool operator==(other) => other.runtimeType == runtimeType && other.color == color;
+  int get hashCode => color.hashCode;
 }
 
 class IconTheme extends InheritedWidget {