anchor.diff:            } else {
               syncChild(null, oldChild, null);
             }
  -          oldChildrenBottom -= 1;
  -        }
  +        } // else the node was probably moved elsewhere, ignore it
  +        oldChildrenBottom -= 1;
         }
       }
   
  
positive.code:     fn();
      _scheduleBuild();
    }
  }
  
  Set<Component> _dirtyComponents = new Set<Component>();
  
negative.code: 
    void _sync(Widget old, dynamic slot) {
      assert(!_debugChildTaken);
      child = syncChild(child, (old as TagNode)?.child, slot);
      if (child != null) {
        assert(child.parent == this);
        assert(child.renderObject != null);
  