anchor.diff:    dynamic _slot;
   
     /// An integer that is guaranteed to be greater than the parent's, if any.
  +  /// The element at the root of the tree must have a depth greater than 0.
     int _depth;
   
     /// The configuration for this element.
  
positive.code:   Element _child;
    bool _dirty = true;
  
    void _rebuild() {
      assert(_debugLifecycleState == _ElementLifecycle.mounted);
      _dirty = false;
  
negative.code:     assert(_depth == null);
      _parent = parent;
      _slot = slot;
      _depth = _parent != null ? _parent._depth + 1 : 1;
      assert(() { _debugLifecycleState = _ElementLifecycle.mounted; return true; });
    }
  
  