Defaults for jest.fn's type parameters

Summary:
This diff adds defaults to Jest.fn's type params.

An upcoming change to flow will be more restrictive about how we implicitly instantiation type arguments. Since the argument to jest.fn is often omitted, this signature is the single biggest offender of our new rules. To reduce the error diff, I'm adding reasonable defaults so that the mocked function can still behave like a function without adding extra flow errors where there were none before.

Changelog: [Internal]

Reviewed By: SamChou19815

Differential Revision: D40394215

fbshipit-source-id: 2df596b6ba8b54680694e08c432a1e7c94dc2df0

flow/jest.js

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