Convert image_cache to using Futures rather than callbacks

sky/sdk/lib/rendering/box.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' as sky;
 
 import 'package:vector_math/vector_math.dart';
 
 import '../base/debug.dart';
@@ skipping 886 matching lines @@
     src = url;
   }
 
   sky.Image _image;
   String _src;
   String get src => _src;
   void set src (String value) {
     if (value == _src)
       return;
     _src = value;
-    image_cache.load(_src, (result) {
+    image_cache.load(_src).then((result) {
       _image = result;
       if (requestedSize.width == null || requestedSize.height == null)
         markNeedsLayout();
       markNeedsPaint();
     });
   }
 
   Size _requestedSize;
   Size get requestedSize => _requestedSize;
   void set requestedSize (Size value) {
@@ skipping 386 matching lines @@
 
   void defaultPaint(PaintingCanvas canvas, Offset offset) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position + offset);
       child = child.parentData.nextSibling;
     }
   }
 }