firebase-auth-2-get-code.dart

StreamSubscription deepLinkSubscription;

@override
void initState() {
  deepLinkSubscription = getLinksStream().listen((String link) {
    String code = getCodeFromGitHubLink(link);
  }, cancelOnError: true);
  super.initState();
}

String getCodeFromGitHubLink(String link) {
  if(link != null) {
    return link.substring(link.indexOf(RegExp('code=')) + 5);
  } else {
    return "";
  }
}