signInWithPopup method
Authenticates a Firebase client using a popup-based OAuth authentication flow.
If succeeds, returns the signed in user along with the provider's credential.
This method is only available on web based platforms.
Implementation
@override
Future<UserCredential> signInWithPopup(AuthProvider provider) {
maybeThrowException(this, Invocation.method(#signInWithPopup, [provider]));
return _fakeSignIn();
}