anchor.diff:    }
   
     void _scheduleBuild() {
  -    assert(!_isBuilding);
  +    assert(!_debugIsBuilding);
       if (_dirty || !_mounted)
         return;
       _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);
  