## Description

This removes all of the comments that are of the form "so-and-so (must not be null|can ?not be null|must be non-null)" from the cases where those values are defines as non-nullable values.

This PR removes them from the material library.

This was done by hand, since it really didn't lend itself to scripting, so it needs to be more than just spot-checked, I think. I was careful to leave any comment that referred to parameters that were nullable, but I may have missed some.

In addition to being no longer relevant after null safety has been made the default, these comments were largely fragile, in that it was easy for them to get out of date, and not be accurate anymore anyhow.

This did create a number of constructor comments which basically say "Creates a [Foo].", but I don't really know how to avoid that in a large scale change, since there's not much you can really say in a lot of cases.  I think we might consider some leniency for constructors to the "Comment must be meaningful" style guidance (which we de facto have already, since there are a bunch of these).

## Related PRs
- https://github.com/flutter/flutter/pull/134984
- https://github.com/flutter/flutter/pull/134992
- https://github.com/flutter/flutter/pull/134993
- https://github.com/flutter/flutter/pull/134994

## Tests
 - Documentation only change.

* dart fix --apply

* manual fixes

This implements a MenuBar widget that can render a Material menu bar, and a MenuAnchor widget used to create a cascading menu in a region. The menus are drawn in the overlay, while the menu bar itself is in the regular widget tree. Keyboard traversal works between the two.

This implementation of the MenuBar uses MenuAnchor to create a cascading menu that contains widgets representing the menu items. These menu items can be any kind of widget, but are typically SubmenuButtons that host submenus, or MenuItemButtons that have shortcut hints (but don't actually activate the shortcuts) and don't host submenus.

Cascading menus can be created outside of a MenuBar by using a MenuAnchor. They can be either given a specific location to appear (a coordinate), or they can be located by the MenuAnchor region that wraps the control that opens them.

The developer may also create a MenuController to pass to the various menu primitives (MenuBar or MenuAnchor) to associate menus so that they can be traversed together and closed together. Creating a controller is not required.