anchor.diff:    void insert(RenderObjectWrapper child, dynamic slot);
   
     void _sync(UINode old, dynamic slot) {
  +    assert(parent != null);
       if (old == null) {
         root = createNode();
         assert(root != null);
  
positive.code:   // where to put this descendant
  
    void remove() {
      _defunct = true;
      root = null;
      handleRemoved();
    }
    void handleRemoved() { }
  
    UINode findAncestor(Type targetType) {
      var ancestor = _parent;
  
negative.code:       oldBuilt = oldComponent._built;
      }
  
      int lastOrder = _currentOrder;
      _currentOrder = _order;
      _currentlyBuilding = this;
      _built = build();
      assert(_built != null);
      _currentlyBuilding = null;
      _currentOrder = lastOrder;
  
      _built = syncChild(_built, oldBuilt, slot);
      assert(_built != null);
      _dirty = false;
      root = _built.root;
      assert(root != null);
    }
  
    void _buildIfDirty() {
      if (!_dirty || !_mounted)
        return;
  
      assert(root != null);
      _sync(null, _slot);
    }
  