signIn method
Implementation
@override
Future<bool> signIn(FirebaseApp app, AuthProvider provider,
{bool isPopup = false}) async {
if (provider is! T) return false;
if (provider.providerId != providerId) return false;
var credential = await directSignIn(app, provider);
if (credential == null) return false;
_authCredentials[app.name] = credential;
return true;
}