samples/poppopwin: upgrade to first-class sample

samples/third_party/pop-pop-win/lib/src/canvas/new_game_element.dart

-part of ppw_canvas;
-
-class NewGameElement extends Thing {
-  final EventHandle<EventArgs> _clickedEvent =
-      new EventHandle<EventArgs>();
-
-  NewGameElement() : super(294, 92) {
-    MouseManager.setClickable(this, true);
-    MouseManager.getClickStream(this).listen((args) =>
-        _clickedEvent.add(EventArgs.empty));
-    Mouse.isMouseDirectlyOverProperty.getStream(this).listen(_mouseDirectlyOver);
-  }
-
-  Stream<EventArgs> get clicked => _clickedEvent.stream;
-
-  void drawOverride(CanvasRenderingContext2D ctx) {
-    final texture = Mouse.isMouseDirectlyOver(this) ?
-        'button_new_game_clicked.png' : 'button_new_game.png';
-    _textureData.drawTextureKeyAt(ctx, texture);
-  }
-
-  GameElement get _gameElement => (parent as CanvasThing).parent;
-
-  TextureData get _textureData => _gameElement._textureData;
-
-  Game get _game => _gameElement._game;
-
-  void _mouseDirectlyOver(args) {
-    invalidateDraw();
-  }
-}