testJson method Null safety

Implementation

Future<void> testJson() async {
  final Model<Operation> m = newModel();
  final String s = json.encoder.convert(m.toJson());
  m.decodeJson(json.decoder.convert(s) as Map<String, dynamic>,
      needsSave: false);
  await expect(
      json.encoder.convert(m.toJson()), s); // It's not much of a test :-)
}