anchor.diff:    }
   
     void _scheduleBuild() {
  -    assert(!_isBuilding);
  +    assert(!_debugIsBuilding);
       if (_dirty || !_mounted)
         return;
       _dirty = true;
  
positive.code:       _ancestor = old._ancestor;
        assert(_renderObject != null);
      }
      if (inDebugBuild) {
        try {
          throw null;
        } catch (_, stack) {
          _renderObject.debugExceptionContext = stack;
        }
      }
      assert(_renderObject == renderObject); // in case a subclass reintroduces it
      assert(renderObject != null);
      assert(mounted);
  
negative.code:     _child = syncChild(_child, oldChild, slot);
      assert(_child != null);
      assert(_child.parent == this);
  
      assert(() {
        _debugIsBuilding = false;
        _debugLastComponent = _debugPreviousComponent;
        return true;
      });
  
      _dirty = false;
      _renderObject = _child.renderObject;
  