First pass at polyfill

[avolkovi]

I had to refactor our AuthImpl a lot to get this to work, TLDR we have:

types:

• externs.Auth (new public types)
• compat.FirebaseAuth (old public types)
• auth-exp/AuthCore (at most externs.Auth | compat.FirebaseAuth, but in reality it's just pared down to the core fields)
• auth-exp/Auth extends auth-exp/AuthCore (auth-exp/AuthCore + private methods)

implementations:

• AuthImplCompat implements auth-exp/Auth
• AuthImpl extends AuthImplCompat implements externs.Auth
• auth-compat-exp/Auth extends AuthImplCompat implements compat.FirebaseAuth

notes:

• I changed most usage of auth-exp/Auth -> auth-exp/AuthCore, since it's now a strict subset of externs.Auth we can drop a ton of casts
• Tests should only use TestAuth since it implements both auth-exp/Auth and externs.Auth and doesn't require casting to either
• The few places we have that actually depend on private methods in auth-exp/Auth should use the unsafe _castAuth and ensure they never call setPersistence, onAuthStateChanged and onIdTokenStateChanged since those methods could have different signatures depending on whether an instance of AuthImpl or auth-compat-exp/Auth was passed in.
• We need to perform an unsafe cast of auth-compat-exp/Auth -> externs.Auth before calling from the polyfill to the new implementation, this cast is unsafe since the signature on the 3 above mentioned changes, hence we must avoid ever calling those methods directly
• AuthImplCompat is generic so currentUser types correctly to the right implementation
• AuthImplCompat constructor takes a UserProvider so it can instantiate users of the right class

heuristics to follow:

• If you are taking input from a user, use externs.Auth
• If you are a private method, use auth-exp/AuthCore (true for 99% cases that only need auth for auth.name), otherwise use auth-exp/Auth if you or a dependency call one of the _ methods on auth
• If you are a test, use TestAuth
• Converting from externs.Auth -> auth-exp/AuthCore is free
• Converting from externs.Auth -> auth-exp/Auth is achieved through an unsafe cast by calling _castAuth()
• Converting from auth-compat-exp/Auth -> externs.Auth is achieved through an unsafe cast by calling this.asExtern()
• Never ever ever call setPersistence, onAuthStateChanged or onIdTokenStateChanged from our code since you have no idea which implementation you're holding on to, use the equivalent _ prefixed methods if you need to call them. Only the user (or integration tests) get to call these directly.

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[sam-gc]

Why not just have all of these in the switch statements call Provider.credentialFromResult() like you do for phone auth

[avolkovi]

good idea, I guess we didn't have these before and I was just copying the old implementation... we should add one to OAuthProvider for the last bit here

[google-oss-bot]

Binary Size Report

Affected SDKs

No changes between base commit (937f335) and head commit (53d294d).

Test Logs

• Base (937f3354): https://github.com/firebase/firebase-js-sdk/actions/runs/187719960
• Head (53d294da): https://github.com/firebase/firebase-js-sdk/actions/runs/193706307