anchor.diff:  
       // if we get here, there is no win
       return null;
  +  }
  +
  +  String move(int square, String player) {
  +    _board[square] = player;
  +    _moveCount++;
  +    return getWinner();
     }
   
     bool get isFull => _moveCount >= 9;
  
positive.code: 
    TTTBoard() {
      _board = new List<String>.filled(9, null);
    }
  
    String move(int square, String player) {
      _board[square] = player;
      _moveCount++;
      return getWinner();
    }
  
    String getWinner() {
  
negative.code:     const [1, 4, 7], // col 2
      const [2, 5, 8], // col 3
      const [0, 4, 8], // diag 1
      const [2, 4, 6]  // diag 2
    ];
  
    List<String> _board;
  