JS: Switch to nullthrows Package [1/4]

Summary:
Switches to the `nullthrows` package instead of using `fbjs/lib/nullthrows`.

The version of `nullthrows` in `fbjs` is outdated and already missing features that exist in the standalone `nullthrows` package.

Also, this mitigates the inevitable collision between `nullthrows` (as a Haste module) and `nullthrows` (as a `node_modules` dependency).

Reviewed By: zertosh

Differential Revision: D9733178

fbshipit-source-id: 1b589d48c1ed57cebf2088b796ad72e212534c0a

flow/fbjs.js

@@ -14,7 +14,3 @@ declare module 'fbjs/lib/invariant' {
     ...args: Array<any>
   ) => void;
 }
-
-declare module 'fbjs/lib/nullthrows' {
-  declare module.exports: <T>(value: ?T) => T;
-}