1. @override
Future<AuthCode> fetchAuthCodeByCode(AuthServer server, String code)

Asks this instance to retrieve an auth code from provided code.

This must return an instance of AuthCode if one exists for code, and null otherwise.

Source 

@override
Future<AuthCode> fetchAuthCodeByCode(AuthServer server, String code) async {
  var query = new Query<ManagedToken>(context)..where.code = code;

  var storage = await query.fetchOne();
  return storage?.asAuthCode();
}