dillon1000/react

Fix for Flow issues in SimpleCacheProvider (#12942)

* Fix for Flow issues in SimpleCacheProvider

**what is the change?:**
- Fixed some flow errors which were somehow swallowed when CI
originally
- Loosen flow types to avoid issue with recursive loop in Flow; https://github.com/facebook/flow/issues/5870

**why make this change?:**
To unbreak master and unblock other changes we want to make.

**test plan:**
Flow passes!

**issue:**
https://github.com/facebook/react/issues/12941

* Fix lints