anchor.diff:        return;
   
       _trace('$_key rebuilding...');
  -    _sync(null, _root.parent, _root.parent.childAfter(_root));
  +    assert(_root != null);
  +    _sync(null, _slot);
     }
   
     void scheduleBuild() {
  
positive.code:     assert(!_debugHasDuplicateIds());
    }
  
    void _remove() {
      assert(children != null);
      for (var child in children) {
  
negative.code:   }
  
    // Returns the child which should be retained as the child of this node.
    UINode _syncChild(UINode node, UINode oldNode, dynamic slot) {
      assert(node != null);
      assert(oldNode == null || node._key == oldNode._key);
  
      if (node == oldNode) {
  