Convert image_cache to using Futures rather than callbacks

sky/sdk/example/game/lib/image_map.dart

 part of sprites;
 
 typedef void ImageMapCallback(ImageMap preloader);
 
 class ImageMap {
 
   Map<String, Image> _images;
 
   int _totalNumImages = 0;
   int _numLoadedImages = 0;
 
   ImageMapCallback _callback;
 
   ImageMap(List<String> urls, ImageMapCallback this._callback) {
     _images = new Map();
     _totalNumImages = urls.length;
     urls.forEach(_addURL);
   }
 
   void _addURL(String url) {
-    image_cache.load(url, (Image image) {
+    image_cache.load(url).then((Image image) {
       // Store reference to image
       _images[url] = image;
 
       // Check if all images are loaded
       _numLoadedImages++;
       if (_numLoadedImages==_totalNumImages) {
         // Everything loaded, make callback
         _callback(this);
       }
     });
   }
 
   Image getImage(String url) => _images[url];
 
   Image operator [](String url) => _images[url];
-}
+}