Add border to notched bottom nav bar Flutter?

Anyone know how I can slap a nice top border on the bottom nav bar?

The widget code looks like this, using the default notch.

Widget build(BuildContext context) {
    return Container(
      color: Theme().background,
      child: BottomAppBar(
          shape: CircularNotchedRectangle(),
          color: Theme().secondary,
          child: Row(...),
    );
  } 

If I add a border to the parent container, it runs behind the plus button like so - I would like the border to follow the notch instead if possible.