anchor.diff:  
     TTTBoard() {
       _board = new List<String>.filled(9, null);
  -  }
  -
  -  String move(int square, String player) {
  -    _board[square] = player;
  -    _moveCount++;
  -    return getWinner();
     }
   
     String getWinner() {
  
positive.code: }
  
  class TTTBoard {
    // win patterns
    static const List<List<int>> WIN_PATTERNS = const [
      const [0, 1, 2], // row 1
      const [3, 4, 5], // row 2
  
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;
  