Convert image_cache to using Futures rather than callbacks

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

Index: sky/sdk/example/game/lib/image_map.dart
diff --git a/sky/sdk/example/game/lib/image_map.dart b/sky/sdk/example/game/lib/image_map.dart
index 577df2a4e0bf10dbd933f14da08ae7171f02a56c..7f0185c8fb0dc1ae9f7df7dc0cd7d3929f05cf97 100644
--- a/sky/sdk/example/game/lib/image_map.dart
+++ b/sky/sdk/example/game/lib/image_map.dart
@@ -18,7 +18,7 @@ class ImageMap {
   }
 
   void _addURL(String url) {
-    image_cache.load(url, (Image image) {
+    image_cache.load(url).then((Image image) {
       // Store reference to image
       _images[url] = image;
 
@@ -34,4 +34,4 @@ class ImageMap {
   Image getImage(String url) => _images[url];
 
   Image operator [](String url) => _images[url];
-}
+}