samples/poppopwin: upgrade to first-class sample

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

 part of ppw_canvas;
 
 class SquareElement extends Thing {
   static const int _size = 80;
 
   static const List<String> _balloonBits = const['balloon_pieces_a.png',
                                                  'balloon_pieces_b.png',
                                                  'balloon_pieces_c.png',
                                                  'balloon_pieces_d.png'];
 
@@ skipping 60 matching lines @@
 
   BoardElement get _board {
     final BoardElement p = this.parent;
     return p;
   }
 
   TextureData get _textureData => _board._textureData;
 
   Game get _game => _board._game;
 }