anchor.diff:  
   import 'package:analyzer/dart/element/element.dart';
   import 'package:build/build.dart';
  -import 'package:json_annotation/json_annotation.dart';
   import 'package:source_gen/source_gen.dart';
   
   import 'helper_core.dart';
  
positive.code: }
  
  abstract class DecodeHelper implements HelperCore {
    final StringBuffer _buffer = StringBuffer();
  
    CreateFactoryResult createFactory(Map<String, FieldElement> accessibleFields,
        Map<String, String> unavailableReasons) {
      assert(config.createFactory);
      assert(_buffer.isEmpty);
  
      final mapType = config.anyMap ? 'Map' : 'Map<String, dynamic>';
      _buffer.write('$targetClassReference '
          '${prefix}FromJson${genericClassArgumentsImpl(true)}'
          '($mapType json) {\n');
  
  
negative.code:         fieldKeyMapArg = ', fieldKeyMap: const $mapLiteral';
        }
  
        buffer..write(fieldKeyMapArg)..write(')');
      } else {
        buffer..write(checks)..write('''
    return ${data.content}''');
        for (final field in data.fieldsToSet) {
          buffer
            ..writeln()
            ..write('    ..$field = ')
            ..write(deserializeFun(field));
        }
      }
      buffer..writeln(';\n}')..writeln();
  
      return CreateFactoryResult(buffer.toString(), data.usedCtorParamsAndFields);
    }
  
    String _checkKeys(Iterable<FieldElement> accessibleFields) {
      final args = <String>[];
  
      String constantList(Iterable<FieldElement> things) =>
          'const ${jsonLiteralAsDart(things.map(nameAccess).toList())}';
  
      if (config.disallowUnrecognizedKeys) {
        final allowKeysLiteral = constantList(accessibleFields);
  
        args.add('allowedKeys: $allowKeysLiteral');
  