Adds support for zOrder and references to parent nodes in sprites

sky/examples/game/main.dart

Index: sky/examples/game/main.dart
diff --git a/sky/examples/game/main.dart b/sky/examples/game/main.dart
index e3a76b341fec4ac0bed3c07a7ab9f1b1b422b1fa..35cb8bdacc49399fd5663b3acee0363f67022736 100644
--- a/sky/examples/game/main.dart
+++ b/sky/examples/game/main.dart
@@ -17,5 +17,7 @@ void main() {
 
 void allLoaded(ImageMap loader) {
   // Create a new app with the sprite box that contains our game world
-  app = new AppView(new GameBox(new GameWorld(loader)));
+  //app = new AppView(new GameBox(new GameWorld(loader)));
+  //print("hello");
+  app = new AppView((new SpriteBox(new GameTests(loader), SpriteBoxTransformMode.letterbox)));
 }