Summary:
The function is not annotated with `const` so `plain()` will return a non-const
reference to the undecorated Runtime already.  Seems like the const_cast was a
hold-over from a previous iteration.

Reviewed By: mhorowitz

Differential Revision: D16016320

fbshipit-source-id: 3dfa1e9acf2fc5c1ad61c9a8cd27c3c2e42036d3