signInAnonymously method
Asynchronously signs in as an anonymous user.
Implementation
//
// If there is already an anonymous user signed in, that user will be
// returned; otherwise, a new anonymous user identity will be created and
// returned.
Future<UserCredential> signInAnonymously() =>
handleThenable(jsObject.signInAnonymously())
.then((u) => UserCredential.fromJsObject(u));