Fixes loading of assets in demo game

examples/game/lib/main.dart

@@ -45,6 +45,17 @@ main() async {
     'assets/game_ui.png',
   ]));
 
+  // TODO(eseidel): SoundEffect doesn't really do anything except hold a future.
+  _sounds['explosion'] = new SoundEffect(_bundle.load('assets/explosion.wav'));
+  _sounds['laser'] = new SoundEffect(_bundle.load('assets/laser.wav'));
+
+  loads.addAll([
+    _sounds['explosion'].load(),
+    _sounds['laser'].load(),
+  ]);
+
+  await Future.wait(loads);
+
   // TODO(eseidel): These load in serial which is bad for startup!
   String json = await _bundle.loadString('assets/sprites.json');
   _spriteSheet = new SpriteSheet(_imageMap['assets/sprites.png'], json);
@@ -54,20 +65,12 @@ main() async {
 
   _app = new GameDemoApp();
 
-  // TODO(eseidel): SoundEffect doesn't really do anything except hold a future.
-  _sounds['explosion'] = new SoundEffect(_bundle.load('assets/explosion.wav'));
-  _sounds['laser'] = new SoundEffect(_bundle.load('assets/laser.wav'));
-
-  loads.addAll([
-    _sounds['explosion'].load(),
-    _sounds['laser'].load(),
-  ]);
+  assert(_spriteSheet.image != null);
 
   SoundTrackPlayer stPlayer = SoundTrackPlayer.sharedInstance();
   SoundTrack music = await stPlayer.load(_bundle.load('assets/temp_music.aac'));
   stPlayer.play(music);
 
-  await Future.wait(loads);
   runApp(_app);
 }
 
@@ -245,6 +248,8 @@ class MainScreenBackground extends NodeWithSize {
     sprtBackground.position = new Point(512.0, 512.0);
     addChild(sprtBackground);
 
+    assert(_spriteSheet.image != null);
+
     StarField starField = new StarField(_spriteSheet, 200, true);
     addChild(starField);
   }