what does @protected mean in dart

As the dev doc says, Dart doesn't have the keywords public , protected , and private . If an identifier starts with an underscore (_), it's private to its library. But I found many @protected keywords in Flutter framework. What does the @protected mean?

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);
}