anchor.diff:  
     @override
     void removeRenderObjectChild(RenderObject child, _AlertSections slot) {
  -    assert(child == renderObject!.contentSection || child == renderObject!.actionsSection);
  -    if (renderObject!.contentSection == child) {
  -      renderObject!.contentSection = null;
  -    } else if (renderObject!.actionsSection == child) {
  -      renderObject!.actionsSection = null;
  +    assert(child == renderObject.contentSection || child == renderObject.actionsSection);
  +    if (renderObject.contentSection == child) {
  +      renderObject.contentSection = null;
  +    } else if (renderObject.actionsSection == child) {
  +      renderObject.actionsSection = null;
       }
     }
   
  
positive.code:     assert(slot != null);
      switch (slot) {
        case _AlertSections.contentSection:
          renderObject!.contentSection = child as RenderBox;
          break;
        case _AlertSections.actionsSection:
          renderObject!.actionsSection = child as RenderBox;
          break;
      }
    }
  
negative.code:   _CupertinoAlertRenderWidget get widget => super.widget as _CupertinoAlertRenderWidget;
  
    @override
    _RenderCupertinoAlert get renderObject => super.renderObject as _RenderCupertinoAlert;
  
    @override
    void visitChildren(ElementVisitor visitor) {
  