anchor.diff:        : _order = _currentOrder + 1,
           super._withKey(key);
   
  -  bool _isBuilding = false;
  +  bool _debugIsBuilding = false;
  +  static String _debugLastComponent;
   
     bool _dirty = true;
   
  
positive.code:   }
  
    void _scheduleBuild() {
      assert(!_isBuilding);
      if (_dirty || !_mounted)
        return;
      _dirty = true;
  
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;
  