https://github.com/FirebaseExtended/flutterfire/blob/master/packages/cloud_firestore/cloud_firestore_platform_interface/lib/src/method_channel/method_channel_firestore.dart#L149-L154

firestore_publish_setting.dart

@override
Future<void> settings({
    bool persistenceEnabled,
    String host,
    bool sslEnabled,
    int cacheSizeBytes,
}) async {
    await channel.invokeMethod<void>('Firestore#settings', <String, dynamic>{
      'app': app.name,
      'persistenceEnabled': persistenceEnabled,
      'host': host,
      'sslEnabled': sslEnabled,
      'cacheSizeBytes': cacheSizeBytes,
    });
}