exact-by-default in xplat/js

Summary:
Turns on the exact-by-default flag in xplat/js and gets rid of the implicit inexact object lint, which has no effect when exact-by-default is on.

Will land this on Monday along with an announcement post.

Changelog: [Internal]

Reviewed By: gkz

Differential Revision: D18863276

fbshipit-source-id: 07a31e957bea1d1e053e8fa5975fdb1b6da1bdc4