Adds better explosions, shield, and game over

sky/sdk/example/game/main.dart

 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/widget.dart';
 import 'package:sky/mojo/net/fetch.dart';
 
 import 'lib/game_demo.dart';
 import 'lib/sprites.dart';
 
 void main() {
   // Load images
   new ImageMap([
       "res/nebula.png",
       "res/sprites.png",
       "res/starfield.png",
     ],
     allImagesLoaded);
 }
 
 void allImagesLoaded(ImageMap loader) {
   _loader = loader;
 
   fetchBody("res/sprites.json").then((Response response) {
     String json = response.bodyAsString();
     _spriteSheet = new SpriteSheet(_loader["res/sprites.png"], json);
     allResourcesLoaded();
   });
 }
 
+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(
 //            new RaisedButton(child:new Text("Hello")),
 //            key: 1
 //          ),
 //          new FlexExpandingChild(
 //            new RaisedButton(child:new Text("Foo!")),
 //            key: 2
 //          )
 //        ]),
 //        right:0.0,
 //        top: 20.0
 //      )
     ]);
   }
 }
 
+void resetGame() {
+  _app.scheduleBuild();
+}
+
 ImageMap _loader;
 SpriteSheet _spriteSheet;