message: Fix possible correctness bug in VectorTree.
  After erase(), the iterator to use it is invalid. Use
  the new location it returns for the subsequent insert().
diff: @@ -758,8 +758,8 @@ class VectorTree : private _VectorTreeImpl {
  adopted_grandchildren.swap(Children()[i].Children());
  {
  // Remove i'th child.
  - const auto insert_iter = children_.begin() + i;
  - children_.erase(insert_iter);
  + const auto remove_iter = children_.begin() + i;
  + const auto insert_iter = children_.erase(remove_iter);
  // Allocate room for new children.
  this_type dummy_node;
  children_.insert(insert_iter, adopted_grandchildren.size(), dummy_node);
  