anchor.diff:  /// Elements can, in principle, have children. Only subclasses of
   /// RenderObjectElement are allowed to have more than one child.
   abstract class Element implements BuildContext {
  +  /// Creates an element that instantiates the given widget.
     Element(Widget widget) : _widget = widget {
       assert(widget != null);
     }
  
positive.code:   }
  
    @override
    void update(_ProxyWidget newWidget) {
      _ProxyWidget oldWidget = widget;
      assert(widget != null);
      assert(widget != newWidget);
      super.update(newWidget);
  
negative.code: }
  
  /// Base class for instantiations of [InheritedWidget] subclasses.
  class InheritedElement extends ProxyElement {
    InheritedElement(InheritedWidget widget) : super(widget);
  
    @override
  