Decouple Canvas from DisplayList and map Picture and PictureRecorder more directly to their Skia co…

sky/examples/raw/hello_world.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // 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';
 
 Picture draw(int a, int r, int g, int b) {
   double width = view.width;
   double height = view.height;
 
-  PictureRecorder recorder = new PictureRecorder(width, height);
+  PictureRecorder recorder = new PictureRecorder();
+  Canvas canvas = recorder.beginRecording(width, height);
   double radius = min(width, height) * 0.45;
 
   Paint paint = new Paint()..color = new Color.fromARGB(a, r, g, b);
-  recorder.drawCircle(width / 2, height / 2, radius, paint);
+  canvas.drawCircle(width / 2, height / 2, radius, paint);
   return recorder.endRecording();
 }
 
 bool handleEvent(Event event) {
   if (event.type == "pointerdown") {
     view.picture = draw(255, 0, 0, 255);
     view.scheduleFrame();
     return true;
   }
 
@@ skipping 11 matching lines @@
   return false;
 }
 
 void main() {
   print("Hello, world");
   view.picture = draw(255, 0, 255, 0);
   view.scheduleFrame();
 
   view.setEventCallback(handleEvent);
 }