anchor.diff:        callback: (Map<String, String> parameters) async {
           if (parameters.containsKey('enabled'))
             await setter(parameters['enabled'] == 'true');
  -        return <String, String>{ 'enabled': await getter() ? 'true' : 'false' };
  +        return <String, dynamic>{ 'enabled': await getter() ? 'true' : 'false' };
         }
       );
     }
  
positive.code:       name: name,
        callback: (Map<String, String> parameters) async {
          await callback();
          return <String, String>{};
        }
      );
    }
  
negative.code:       callback: (Map<String, String> parameters) async {
          if (parameters.containsKey('value'))
            await setter(parameters['value']);
          return <String, dynamic>{ 'value': await getter() };
        }
      );
    }
  