正如开发文档所说，Dart doesn't have the keywords public,protected,and private . If an identifier starts with an underscore (_),it's private to its library.但是我在Flutter框架中发现了许多@protected关键字。 @protected是什么意思？

abstract class InheritedWidget extends ProxyWidget {
  const InheritedWidget({ Key key,Widget child })
    : super(key: key,child: child);

  @override
  InheritedElement createElement() => InheritedElement(this);

  @protected
  bool updateShouldNotify(covariant InheritedWidget oldWidget);
}