How do I pass non-string data to a named route in Flutter?

EDIT:

It is now possible to pass complex arguments to Navigator.pushNamed:

String id;
Navigator.pushNamed(context, "https://stackoverflow.com/users", arguments: id);

It can then be used within onGenerateRoute to customize route building with these arguments:

MaterialApp(
  title: 'Flutter Hooks Gallery',
  onGenerateRoute: (settings) {
    final arguments = settings.arguments;
    switch (settings.name) {
      case "https://stackoverflow.com/users":
        if (arguments is String) {
          // the details page for one specific user
          return UserDetails(arguments);
        }
        else {
          // a route showing the list of all users
          return UserList();
        }
      default:
        return null;
    }
  },
);