signInWithCustomToken method
Asynchronously signs in using a custom token.
Custom tokens are used to integrate Firebase Auth with existing auth systems, and must be generated by the auth backend.
Fails with an error if the token is invalid, expired, or not accepted by the Firebase Auth service.
Implementation
Future<UserCredential> signInWithCustomToken(String token) =>
handleThenable(jsObject.signInWithCustomToken(token))
.then((u) => UserCredential.fromJsObject(u));