anchor.diff:    static Component _currentlyBuilding;
     List<Function> _mountCallbacks;
     List<Function> _unmountCallbacks;
  +  dynamic _slot; // cached slot from the last time we were synced
   
     void onDidMount(Function fn) {
       if (_mountCallbacks == null)
  
positive.code:     }
    }
  
    void _sync(UINode old, RenderCSSContainer host, RenderCSS insertBefore) {
      for (var type in listeners.keys) {
        _ensureDocumentListener(type);
      }
  
      super._sync(old, host, insertBefore);
    }
  }
  
  
negative.code:       return;
  
      _trace('$_key rebuilding...');
      assert(_root != null);
      _sync(null, _slot);
    }
  
    void scheduleBuild() {
  