Adds better explosions, shield, and game over

sky/sdk/example/game/main.dart

Index: sky/sdk/example/game/main.dart
diff --git a/sky/sdk/example/game/main.dart b/sky/sdk/example/game/main.dart
index 22648ab2e5d9d6302b7adc7d007e2f810f34b35d..691fb249caa627bd271cc75f115450e23002c1a2 100644
--- a/sky/sdk/example/game/main.dart
+++ b/sky/sdk/example/game/main.dart
@@ -25,15 +25,18 @@ void allImagesLoaded(ImageMap loader) {
   });
 }
 
+GameDemoApp _app;
+
 void allResourcesLoaded() {
-  runApp(new GameDemoApp());
+  _app = new GameDemoApp();
+  runApp(_app);
 }
 
 class GameDemoApp extends App {
 
   Widget build() {
     return new Stack([
-      new SpriteWidget(new GameDemoWorld(_loader, _spriteSheet)),
+      new SpriteWidget(new GameDemoWorld(_app, _loader, _spriteSheet)),
 //      new StackPositionedChild(
 //        new Flex([
 //          new FlexExpandingChild(
@@ -52,5 +55,9 @@ class GameDemoApp extends App {
   }
 }
 
+void resetGame() {
+  _app.scheduleBuild();
+}
+
 ImageMap _loader;
 SpriteSheet _spriteSheet;