Either support converting to converter arg types or explicitly disallow non-JSON argument types

See https://github.com/google/json_serializable.dart/issues/795

Converter takes type of `List`.

generates

```dart
LoginState _$LoginStateFromJson(Map json) {
return LoginState(
cookies:
const _CookiesJsonConverter().fromJson(json['cookies'] as List),
);
}
```

Of course the `as List` will fail always.

We could/should do better here!

Annoying, but this is not a blocker. User can just change the converter to do the right thing.