Back out "Back out "Revert D18542342: Boxed variable dispatch""
Summary: Original commit changeset: 7f3e32a6ee0c
Test Plan: waitforsandcastle
Reviewed By: ezyang
Differential Revision: D18766763
fbshipit-source-id: 51bb7aac7cb7ce3df94681e838949e7a156e3ad9
