samples/poppopwin: upgrade to first-class sample

samples/pop_pop_win/lib/src/canvas/game_background_element.dart

 part of ppw_canvas;
 
 class GameBackgroundElement extends Thing {
   GameBackgroundElement() : super(0, 0) {
     cacheEnabled = true;
   }
 
   void update() {
     if(size != this._gameElement.size) {
       size = this._gameElement.size;
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   void _drawCorner(CanvasRenderingContext2D ctx) {
     _textureData.drawTextureKeyAt(ctx, 'background_top_left.png');
     _textureData.drawTextureKeyAt(ctx, 'background_side_left.png',
         new Coordinate(0, GameElement._boardOffset.y));
   }
 
   GameElement get _gameElement => (parent as CanvasThing).parent;
 
   TextureData get _textureData => _gameElement._textureData;
 }