Convert image_cache to using Futures rather than callbacks

sky/sdk/example/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:math' as math;
 import 'dart:sky';
 
 import 'package:sky/mojo/net/image_cache.dart' as image_cache;
 
 double timeBase = null;
@@ skipping 41 matching lines @@
     print("Existing image was loaded again");
   }
 }
 
 bool handleEvent(Event event) {
   if (event.type == "pointerdown") {
     return true;
   }
 
   if (event.type == "pointerup") {
-    image_cache.load(url2, handleImageLoad);
+    image_cache.load(url2).then(handleImageLoad);
     return true;
   }
 
   return false;
 }
 
 void main() {
-  image_cache.load(url1, handleImageLoad);
-  image_cache.load(url1, handleImageLoad);
+  image_cache.load(url1).then(handleImageLoad);
   view.setEventCallback(handleEvent);
   view.setBeginFrameCallback(beginFrame);
 }