samples/poppopwin: upgrade to first-class sample

samples/pop_pop_win/lib/src/poppopwin/game.dart

 part of ppw;
 
 class Game {
   final Field field;
   final Array2d<SquareState> _states;
   final EventHandle<EventArgs> _updatedEvent = new EventHandle<EventArgs>();
   final EventHandle<GameState> _gameStateEvent = new EventHandle<GameState>();
 
   GameState _state;
   int _bombsLeft;
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   int _getAdjacentCount(int x, int y, SquareState state) {
     int val = 0;
     for(final i in field.getAdjacentIndices(x, y)) {
       if(_states[i] == state) {
         val++;
       }
     }
     return val;
   }
 }