Move build() off microtasks

Rather than using a microtask to schedule component build functions, instead
use the scheduler. We now tread building just like layout and painting as a
visual update.

Author: abarth

sky/packages/sky/lib/rendering/sky_binding.dart

@@ -47,7 +47,7 @@ class SkyBinding {
       _renderView = renderViewOverride;
     }
     assert(_renderView != null);
-    scheduler.addPersistentFrameCallback(_beginFrame);
+    scheduler.addPersistentFrameCallback(beginFrame);
 
     assert(_instance == this);
   }
@@ -69,7 +69,7 @@ class SkyBinding {
   void set root(RenderBox value) {
     _renderView.child = value;
   }
-  void _beginFrame(double timeStamp) {
+  void beginFrame(double timeStamp) {
     RenderObject.flushLayout();
     RenderObject.flushPaint();
     _renderView.paintFrame();

sky/packages/sky/lib/widgets/widget.dart

@@ -7,6 +7,7 @@ import 'dart:collection';
 import 'dart:sky' as sky;
 
 import 'package:sky/base/hit_test.dart';
+import 'package:sky/base/scheduler.dart' as scheduler;
 import 'package:sky/mojo/activity.dart' as activity;
 import 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/object.dart';
@@ -759,6 +760,9 @@ void _absorbDirtyComponents(List<Component> list) {
 }
 
 void _buildDirtyComponents() {
+  if (_dirtyComponents.isEmpty)
+    return;
+
   Stopwatch sw;
   if (_shouldLogRenderDuration)
     sw = new Stopwatch()..start();
@@ -805,11 +809,10 @@ void _scheduleComponentForRender(Component c) {
   _dirtyComponents.add(c);
   if (!_buildScheduled) {
     _buildScheduled = true;
-    new Future.microtask(_buildDirtyComponents);
+    scheduler.ensureVisualUpdate();
   }
 }
 
-
 // RenderObjectWrappers correspond to a desired state of a RenderObject.
 // They are fully immutable, with one exception: A Widget which is a
 // Component which lives within an MultiChildRenderObjectWrapper's
@@ -1178,6 +1181,11 @@ class WidgetSkyBinding extends SkyBinding {
     }
   }
 
+  void beginFrame(double timeStamp) {
+    _buildDirtyComponents();
+    super.beginFrame(timeStamp);
+  }
+
 }
 
 abstract class App extends StatefulComponent {