redepthChild method
Adjust the depth of the given child to be greater than this node's own
depth.
Only call this method from overrides of redepthChildren.
Implementation
@protected
void redepthChild(RenderObject child) {
  assert(child.owner == owner);
  if (child._depth <= _depth) {
    child._depth = _depth + 1;
    child.redepthChildren();
  }
}