anchor.diff:        : _order = _currentOrder + 1,
           super._withKey(key);
   
  -  bool _isBuilding = false;
  +  bool _debugIsBuilding = false;
  +  static String _debugLastComponent;
   
     bool _dirty = true;
   
  
positive.code:       oldChild = old._child;
      }
  
      _isBuilding = true;
  
      int lastOrder = _currentOrder;
      _currentOrder = _order;
  
negative.code:       _ancestor = old._ancestor;
        assert(_renderObject != null);
      }
      assert(() {
        _renderObject.debugExceptionContext = Component._debugLastComponent;
        return true;
      });
      assert(_renderObject == renderObject); // in case a subclass reintroduces it
      assert(renderObject != null);
      assert(mounted);
  