anchor.diff:      return new Field._internal(count, cols, rows, squares);
     }
   
  -  Field._internal(this.mineCount, this.cols, this.rows, this._squares) {
  +  Field._internal(this.mineCount, this.cols, this.rows, this._squares) :
  +    this._adjacents = new List<int>() {
       assert(cols > 0);
       assert(rows > 0);
       assert(_squares.length == cols * rows);
  
positive.code: 
    int getAdjacent(int x, int y) {
      final i = _getIndex(x, y);
      return null;
    }
  
    int _getIndex(int x, int y) {
  
negative.code:       }
      }
      assert(count == mineCount);
  
      _adjacents.insertRange(0, _squares.length);
    }
  
    bool isMine(int x, int y) {
  