anchor.diff:      requireArgument(height > 0, 'height');
       final s = new List<T>();
       s.insertRange(0, width * height, initialValue);
  +    assert(s.length == width * height);
       return new Array2d.wrap(width, s);
     }
   
  
positive.code:     return x + y * width;
    }
  }
  
negative.code:   void operator []=(int index, T value) {
      _source[index] = value;
    }
  
    // TODO: test
    //  - especially equality across instances, rows, etc
    ListBase<ListBase<T>> get rows =>
        new _Array2dRows(this);
  
    T get(int x, int y) {
      final i = _getIndex(x, y);
  