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