samples/poppopwin: upgrade to first-class sample

samples/pop-pop-win/lib/src/canvas/score_element.dart

 part of ppw_canvas;
 
 // NOTE: setGameManager must be called immediately after construction
 
 class ScoreElement extends Thing {
   static const _bombsLeftStr = "BOMBS LEFT:";
   static const _valueOffset = 15;
 
   GameManager _gameManager;
 
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   num _getTextSize(CanvasRenderingContext2D ctx) {
     if(_textSize == null) {
       final mets = ctx.measureText(_bombsLeftStr);
       _textSize = mets.width;
     }
     return _textSize;
   }
 
   Game get _game => _gameManager.game;
 }