Adds star field to game start screen

packages/flutter/example/game/main.dart

@@ -90,10 +90,18 @@ class GameDemoApp extends App {
   }
 
   Widget _buildMainScene(navigator, route) {
-    return new Center(
+    return new Stack([
+      new SpriteWidget(new MainScreenBackground()),
+      new Center(
         child: new TextureButton(
           onPressed: () {
-          _game = new GameDemoWorld(_app, navigator, _loader, _spriteSheet, _spriteSheetUI);
+            _game = new GameDemoWorld(
+              _app,
+              navigator,
+              _loader,
+              _spriteSheet,
+              _spriteSheetUI
+            );
             navigator.pushNamed('/game');
           },
           texture: _spriteSheetUI["btn_play_up.png"],
@@ -101,7 +109,8 @@ class GameDemoApp extends App {
           width: 128.0,
           height: 128.0
         )
-    );
+      )
+    ]);
   }
 }
 
@@ -194,3 +203,14 @@ class _TextureButtonToken {
     return value;
   }
 }
+
+class MainScreenBackground extends NodeWithSize {
+  MainScreenBackground() : super(new Size(1024.0, 1024.0)) {
+    Sprite sprtBackground = new Sprite.fromImage(_loader["assets/starfield.png"]);
+    sprtBackground.position = new Point(512.0, 512.0);
+    addChild(sprtBackground);
+
+    StarField starField = new StarField(_spriteSheet, 200, true);
+    addChild(starField);
+  }
+}