Summary:
Changelog: [Internal]
This diff add types to some of the common globals so uses of
them through `global` are now typed.
All the globals are marked as read-only for their intented uses.
However, some of them do have write cites (mostly are in tests to
deliberately set up a special test environment). Those write cites
are considered as "necessary evil" and annotated as `FlowFixMe`.
Reviewed By: yungsters
Differential Revision: D30158145
fbshipit-source-id: 93a99063361a4b7a1e33d9fc97a661be30a4d8f9
e60ad0837e