Convert image_cache to using Futures rather than callbacks

sky/sdk/lib/painting/box_painter.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 'dart:sky' show Point, Offset, Size, Rect, Color, Paint, Path;
 
 import 'shadows.dart';
 import 'package:sky/mojo/net/image_cache.dart' as image_cache;
@@ skipping 122 matching lines @@
 
 class BackgroundImage {
   final String src;
   final BackgroundFit fit;
   final BackgroundRepeat repeat;
   BackgroundImage({
     this.src,
     this.fit: BackgroundFit.scaleDown,
     this.repeat: BackgroundRepeat.noRepeat
   }) {
-    image_cache.load(src, (image) {
+    image_cache.load(src).then((image) {
       if (image == null)
         return;
       _image = image;
       _size = new Size(image.width.toDouble(), image.height.toDouble());
       for (Function listener in _listeners) {
         listener();
       }
     });
   }
 
@@ skipping 219 matching lines @@
     path.close();
     canvas.drawPath(path, paint);
   }
 
   void paint(sky.Canvas canvas, Rect rect) {
     _paintBackgroundColor(canvas, rect);
     _paintBackgroundImage(canvas, rect);
     _paintBorder(canvas, rect);
   }
 }