Summary:
Addresses a performance regression introduced by automatic linting: Compound assignment operators are much slower than keeping assignment separate on `let` bindings in certain versions of v8.
This reverts the relevant changes and configures eslint to *disallow* these operators explicitly in `metro-source-map`.
Reviewed By: mjesun
Differential Revision: D6520485
fbshipit-source-id: 16f35f5cd691ce6b1924480cbc30fbaa1275f730
David Aurelio