anchor.diff:    @override
     String get message => 'These keys had `null` values, '
         'which is not allowed: $keysWithNullValues';
  +
  +  @override
  +  String toString() => 'DisallowedNullValueException: $message';
   }
  
positive.code:   MissingRequiredKeysException(this.missingKeys, Map map)
        : assert(missingKeys.isNotEmpty),
          super._(map);
  }
  
  /// Exception thrown if there are keys with disallowed `null` values in a JSON
  
negative.code:       : super._(map);
  
    @override
    String toString() => 'UnrecognizedKeysException: $message';
  }
  
  /// Exception thrown if there are missing required keys in a JSON map that was
  