Make rendering use PaintingNodes for increased efficiency.

sky/sdk/example/raw/painting_node.dart

Index: sky/sdk/example/raw/painting_node.dart
diff --git a/sky/sdk/example/raw/hello_world.dart b/sky/sdk/example/raw/painting_node.dart
similarity index 59%
copy from sky/sdk/example/raw/hello_world.dart
copy to sky/sdk/example/raw/painting_node.dart
index e667391734b3dcfd9f595a90d706ab33c6d84b98..7297540ee4df303e208aed57ef4c1ee652027343 100644
--- a/sky/sdk/example/raw/hello_world.dart
+++ b/sky/sdk/example/raw/painting_node.dart
@@ -2,17 +2,32 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import "dart:math";
 import 'dart:sky';
 
+PaintingNode paintingNode = null;
 Picture draw(int a, int r, int g, int b) {
+  Rect bounds = new Rect.fromLTRB(0.0, 0.0, view.width, view.height);
   Size size = new Size(view.width, view.height);
 
   PictureRecorder recorder = new PictureRecorder();
-  Canvas canvas = new Canvas(recorder, size);
+  Canvas canvas = new Canvas(recorder, bounds);
   double radius = size.shortestSide * 0.45;
 
   Paint paint = new Paint()..color = new Color.fromARGB(a, r, g, b);
   canvas.drawCircle(size.center(Point.origin), radius, paint);
+
+  if (paintingNode == null) {
+    paintingNode = new PaintingNode();
+    Paint innerPaint = new Paint()..color = new Color.fromARGB(a, 255 - r, 255 - g, 255 - b);
+    PictureRecorder innerRecorder = new PictureRecorder();
+    Canvas innerCanvas = new Canvas(innerRecorder, bounds);
+    innerCanvas.drawCircle(size.center(Point.origin), radius * 0.5, innerPaint);
+
+    paintingNode.setBackingDrawable(innerRecorder.endRecordingAsDrawable());
+  }
+  canvas.drawPaintingNode(paintingNode);
+
   return recorder.endRecording();
 }
 
@@ -29,16 +44,10 @@ bool handleEvent(Event event) {
     return true;
   }
 
-  if (event.type == "back") {
-    print("Pressed back button.");
-    return true;
-  }
-
   return false;
 }
 
 void main() {
-  print("Hello, world");
   view.picture = draw(255, 0, 255, 0);
   view.scheduleFrame();