Switching from annotated to block style for fromJson() method allows @JsonSerializable() annotation usage

[er0ckx]

This came up in #463 as a solution and finally solved my issue; therefore, I think it's worth documenting.

When you need to have both the freezed and JsonSerializer() annotations on your class, your fromJson() factory declaration MUST use block form not annotation form.

// DO THIS
@freezed
@JsonSerializable(fieldRename: FieldRename.snake)
class MapPlace with _$MapPlace {
  const MapPlace._();

  const factory MapPlace({
    ...
  }) = _MapPlace;

  factory MapPlace.fromJson(Map<String, dynamic> json) {
    return _$MapPlaceFromJson(json);
  }
}

  // NOT THIS
  ...
  factory MapPlaceLocation.fromJson(Map<String, Object?> json) => 
    _$MapPlaceLocationFromJson(json);
  ...

I needed to use the "fieldRename" capability of the @JsonSerializer() annotation on a couple of my freezed classes. I kept getting errors like this. But once I changed to block form like described in #463 then everything worked.

lib/maps_service/model_places_nearby.g.dart:9:10: Error: '_$MapPlaceFromJson' is already declared in this scope.
MapPlace _$MapPlaceFromJson(Map<String, dynamic> json) => MapPlace(
         ^^^^^^^^^^^^^^^^^^
lib/maps_service/model_places_nearby.freezed.dart:178:10: Context: Previous declaration of '_$MapPlaceFromJson'.
MapPlace _$MapPlaceFromJson(Map<String, dynamic> json) {
         ^^^^^^^^^^^^^^^^^^
lib/maps_service/model_places_nearby.dart:35:7: Error: Can't use '_$MapPlaceFromJson' because it is declared more than once.
      _$MapPlaceFromJson(json);

Thanks,
Eric