typechecker: some housekeeping in hhi files

Reviewed By: @jwatzman

Differential Revision: D1701098

Signature: t1:1701098:1416873058:f81c48c5748ea4ade05e3d2cbf9b004afea0420e
Silence "unsound type" warning

Summary: No idea why it doesn't know that `exit` is okay, and why it
shows up here and not on line 122, but w/e.

Fixes https://github.com/facebook/flow/issues/116

Reviewed By: @elgenie

Differential Revision: D1701802

Signature: t1:1701802:1416871970:eebc7d06d8a4203b6cd59637cc0648b3ae26d7fa