anchor.diff:        callback: (Map<String, String> parameters) async {
           if (parameters.containsKey(name))
             await setter(double.parse(parameters[name]));
  -        return <String, String>{ name: (await getter()).toString() };
  +        return <String, dynamic>{ name: (await getter()).toString() };
         }
       );
     }
  
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() };
        }
      );
    }
  