Summary:
We have some "internal-only" stuff we want to put in the attribute of a layout node.
This allows us to do so a way that keeps it out of open-source and avoid the clunky subclassing.
Reviewed By: adamjernst
Differential Revision: D24162047
fbshipit-source-id: 518ab9fbf0b4274076f5b0ae1b249e197563fa7f