anchor.diff:  class Game {
     final Field field;
     final List<SquareState> _states;
  +  final EventHandle<EventArgs> _updatedEvent;
   
     GameState _state;
     int _minesLeft;
  
positive.code:   int get revealsLeft => _revealsLeft;
  
    GameState get state => _state;
  
    SquareState getSquareState(int x, int y) {
      final i = field._getIndex(x, y);
  
negative.code:       _states[i] = SquareState.hidden;
        _minesLeft++;
      }
      _update();
    }
  
    void reveal(int x, int y) {
  