anchor.diff:  class FieldTest {
  +  // This grid
  +  // XXXXX2
  +  // X7X8X3
  +  // X5XXX2
  +  // X32321
  +  // 110000
  +
  +  static final sample = const
  +      [null, null, null, null, null, 2,
  +       null,    7, null,    8, null, 3,
  +       null,    5, null, null, null, 2,
  +       null,    3,    2,    3,    2, 1,
  +          1,    1,    0,    0,    0, 0];
  +
  +
  +  static Field getSampleField() {
  +    final bools = new List<bool>.from(sample.map((x) => x == null));
  +
  +    return new Field.fromSquares(6, 5, bools);
  +  }
  +
     static void run() {
       test('defaults', _testDefaults);
       test('mineCount', _testMineCount);
  
positive.code:       for(int y = 0; y < f.rows; y++) {
          final i = x + y * f.cols;
          final adj = f.getAdjacent(x, y);
          expect(adj, equals(source[i]));
        }
      }
    }
  
negative.code:   }
  
    static void _testAdjacent() {
      final f = getSampleField();
  
      expect(f.mineCount, equals(13));
  
  