anchor.diff:  
   }
   
  -abstract class Component extends Widget with _Heir {
  +abstract class Component extends Widget {
   
     Component({ String key, bool stateful })
         : _stateful = stateful != null ? stateful : false,
  
positive.code:     assert(_built != null);
      assert(root != null);
      _built.detachRoot();
    }
  
    bool _retainStatefulNodeIfPossible(Widget old) {
  
negative.code:   void didMount() {
      assert(!_disqualifiedFromEverAppearingAgain);
      super.didMount();
    }
  
    void remove() {
  