anchor.diff:        RenderObject debugPreviousActiveLayout;
         assert(!_debugMutationsLocked);
         assert(!_debugDoingThisLayoutWithCallback);
  +      assert(_debugCanParentUseSize != null);
         assert(() {
           _debugMutationsLocked = true;
  -        _debugCanParentUseSize = false;
           _debugDoingThisLayout = true;
           debugPreviousActiveLayout = _debugActiveLayout;
           _debugActiveLayout = this;
  
positive.code:       assert(() {
          _debugActiveLayout = debugPreviousActiveLayout;
          _debugDoingThisLayout = false;
          _debugCanParentUseSize = null;
          _debugMutationsLocked = false;
          return true;
        });
  
negative.code:     assert(() {
        _debugActiveLayout = debugPreviousActiveLayout;
        _debugDoingThisLayout = false;
        _debugMutationsLocked = false;
        return true;
      });
  