anchor.diff:        return;
   
       _trace('$_key rebuilding...');
  -    _sync(null, _root.parent, _root.parent.childAfter(_root));
  +    assert(_root != null);
  +    _sync(null, _slot);
     }
   
     void scheduleBuild() {
  
positive.code:   RenderCSS _createNode();
    RenderNodeWrapper get _emptyNode;
  
    void _sync(UINode old, RenderCSSContainer host, RenderCSS insertBefore) {
      if (old == null) {
        _root = _createNode();
        assert(_root != null);
        host.add(_root, before: insertBefore);
        old = _emptyNode;
      } else {
        _root = old._root;
  
negative.code: 
      if (node._willSync(oldNode)) {
        _traceSync(_SyncOperation.STATEFUL, node._key);
        oldNode._sync(node, slot);
        node._defunct = true;
        assert(oldNode._root is RenderCSS);
        return oldNode;
  