remove unused arg

Author: kevmoo

json_serializable/lib/src/json_key_helpers.dart

@@ -12,11 +12,10 @@ import 'package:meta/meta.dart';
 import 'package:source_gen/source_gen.dart';
 
 @alwaysThrows
-T _throwUnsupported<T>(FieldElement element, String message, {String todo}) =>
+T _throwUnsupported<T>(FieldElement element, String message) =>
     throw new InvalidGenerationSourceError(
         'Error with `@JsonKey` on `${element.displayName}`. $message',
-        element: element,
-        todo: todo);
+        element: element);
 
 final _jsonKeyExpando = new Expando<JsonKeyWithConversion>();