Summary:Fixes mocking in NavigationExperimental tests and manually mock React to make things pass
Closes https://github.com/facebook/react-native/pull/6124
Differential Revision: D2970955
fb-gh-sync-id: ece45f1193a23a39d49fb9c2f529e1d709189f31
shipit-source-id: ece45f1193a23a39d49fb9c2f529e1d709189f31
Summary: There is a NavigationState type within this module so the name cannot be shared
Reviewed By: hedgerwang
Differential Revision: D2938311
fb-gh-sync-id: c5208755c9dfa5bf0e67666957c01e203ddd4218
shipit-source-id: c5208755c9dfa5bf0e67666957c01e203ddd4218