如何在Dart / Flutter中扩展课程

我有A班：

class A{
    String title;
    String content;
    IconData iconData;
    Function onTab;
    A({this.title, this.content, this.iconData, this.onTab});
}

我如何创建用其他变量扩展类A的类B，如下所示：

class B extends A{
    bool read;
    B({this.read});
}

尝试过但不起作用

let o = new B(
          title: "New notification",
          iconData: Icons.notifications,
          content: "Lorem ipsum doro si maet 100",
          read: false,
          onTab: (context) => {

          });