Fix or expose previously hidden type errors to prepare for LTI mode

Summary:
Changelog:
[Internal]

Differential Revision: D40600946

fbshipit-source-id: d169402eb16204ac62788806d6d3fb900776f3d2

Author: SamChou19815

flow/jest.js

@@ -810,7 +810,7 @@ type JestObjectType = {
    * Returns a new, unused mock function. Optionally takes a mock
    * implementation.
    */
-  fn<TArguments: $ReadOnlyArray<mixed> = $ReadOnlyArray<mixed>, TReturn = any>(
+  fn<TArguments: $ReadOnlyArray<mixed> = $ReadOnlyArray<any>, TReturn = any>(
     implementation?: (...args: TArguments) => TReturn,
   ): JestMockFn<TArguments, TReturn>,
   /**