anchor.diff:        return;
   
       assert(_host != null);
  +    _trace('$_key rebuilding...');
       _sync(null, _host, _insertionPoint);
     }
   
  
positive.code:   Node _parent;
    sky.Node _root;
    bool _defunct = false;
  
    Node({ Object key }) {
      _key = key == null ? "$runtimeType" : "$runtimeType-$key";
  
negative.code:     currentNode = null;
      while (oldStartIndex < oldEndIndex) {
        oldNode = oldChildren[oldStartIndex];
        _removeChild(oldNode);
        advanceOldStartIndex();
      }
    }
  