* 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