In the newest version of firebase_auth:
FirebaseUser has been changed to User
AuthResult has been changed to UserCredential
GoogleAuthProvider.getCredential() has been changed to GoogleAuthProvider.credential()
onAuthStateChanged which notifies about changes to the user's sign-in state was replaced with authStateChanges()
currentUser() which is a method to retrieve the currently logged in user, was replaced with the property currentUser and it no longer returns a Future<FirebaseUser>