Remove the flicker from MimicOverlay

There were two problems:

1) When starting the mimic, we put up a bad frame because although we set the
   `begin` value of the animation, we were building using the current `value`,
   which hadn't been updated.

2) When stoping the mimic, we'd dirty a component during didUnmount, which
   wouldn't get cleaned until the next frame. Now we're sure to clean all the
   components before leaving flushBuild.

packages/flutter/lib/widgets/framework.dart

@@ -116,7 +116,7 @@ abstract class GlobalKey extends Key {
   }
 
   static void _notifyListeners() {
-    assert(!_inRenderDirtyComponents);
+    assert(!_inBuildDirtyComponents);
     assert(!Widget._notifyingMountStatus);
     assert(_debugDuplicates.isEmpty);
     _notifyingListeners = true;
@@ -161,7 +161,7 @@ abstract class Widget {
   }
 
   // you should not build the UI tree ahead of time, build it only during build()
-  bool _isConstructedDuringBuild() => this is AbstractWidgetRoot || this is App || _inRenderDirtyComponents || _inLayoutCallbackBuilder > 0;
+  bool _isConstructedDuringBuild() => this is AbstractWidgetRoot || this is App || _inBuildDirtyComponents || _inLayoutCallbackBuilder > 0;
 
   Key _key;
 
@@ -755,7 +755,7 @@ abstract class StatefulComponent extends Component {
 
 Set<Component> _dirtyComponents = new Set<Component>();
 bool _buildScheduled = false;
-bool _inRenderDirtyComponents = false;
+bool _inBuildDirtyComponents = false;
 int _inLayoutCallbackBuilder = 0;
 
 class LayoutCallbackBuilderHandle { bool _active = true; }
@@ -793,7 +793,8 @@ void _buildDirtyComponents() {
   if (_shouldLogRenderDuration)
     sw = new Stopwatch()..start();
 
-  _inRenderDirtyComponents = true;
+  while (!_dirtyComponents.isEmpty) {
+    _inBuildDirtyComponents = true;
     try {
       sky.tracing.begin('Component.flushBuild');
       List<Component> sortedDirtyComponents = new List<Component>();
@@ -813,11 +814,12 @@ void _buildDirtyComponents() {
       }
     } finally {
       _buildScheduled = false;
-    _inRenderDirtyComponents = false;
+      _inBuildDirtyComponents = false;
       sky.tracing.end('Component.flushBuild');
     }
 
     Widget._notifyMountStatusChanged();
+  }
 
   if (_shouldLogRenderDuration) {
     sw.stop();

packages/flutter/lib/widgets/mimic_overlay.dart

@@ -70,8 +70,9 @@ class MimicOverlay extends AnimatedComponent {
     setState(() {
       // TODO(abarth): We need to convert global bounds into local coordinates.
       _mimicBounds.begin = globalToLocal(globalBounds.topLeft) & globalBounds.size;
-      _expandPerformance.forward();
+      _mimicBounds.value = _mimicBounds.begin;
     });
+    _expandPerformance.forward();
   }
 
   Widget build() {