signUp method Null safety
Signs up a new user.
This will create a new user on the server, and also persist the session.
Implementation
Future<LCUser> signUp() async {
if (isNullOrEmpty(username)) {
throw ArgumentError.notNull('username');
}
if (isNullOrEmpty(password)) {
throw ArgumentError.notNull('password');
}
if (objectId != null) {
throw ArgumentError('Cannot sign up a user that already exists.');
}
await save();
return this;
}