Refactor image handling in Sky

sky/examples/raw/spinning_image.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:sky';
 
 double timeBase = null;
 
-Image image;
+Image image = null;
 
 void beginFrame(double timeStamp) {
   if (timeBase == null) timeBase = timeStamp;
   double delta = timeStamp - timeBase;
   PictureRecorder canvas = new PictureRecorder(view.width, view.height);
   canvas.translate(view.width / 2.0, view.height / 2.0);
   canvas.rotateDegrees(delta / 10);
   canvas.scale(0.2, 0.2);
   Paint paint = new Paint()..setARGB(255, 0, 255, 0);
-  canvas.drawImage(image, -image.width / 2.0, -image.height / 2.0, paint);
+  if (image != null)
+    canvas.drawImage(image, -image.width / 2.0, -image.height / 2.0, paint);
   view.picture = canvas.endRecording();
   view.scheduleFrame();
 }
 
 void main() {
-  image = new Image();
-  image.src = "https://www.dartlang.org/logos/dart-logo.png";
+  new ImageLoader("https://www.dartlang.org/logos/dart-logo.png", (result) {
+    if (result != null) {
+      print("${result.width}x${result.width} image loaded!");
+      image = result;
+      view.scheduleFrame();
+    } else {
+      print("Image failed to load");
+    }
+  }).load();
   view.setBeginFrameCallback(beginFrame);
-  view.scheduleFrame();
 }