FR: please add function Auth.getCurrentUser() as a promise that will return current user or null

Problem

Firebase SDK version: 4.9.0 Firebase Product: auth `Auth.currentUser` object is unreliable to get user from persistence storage. If the authentication system could be pending and don't really have ensure finishing endpoint then you should have a promise for it So I want to have promise function `Auth.getCurrentUser()` in addition to `onAuthStateChange` that would eventually return user but would return null if no user exist in persistence storage or user require sign in from expired token