For some embeddings (macOS, Windows, Linux, ChromeOS, Fuschia), an application-level menu bar widget is necessary. It should be able to display keyboard accelerators, menu item separators, and have cascading menus.

The design doc is here: https://flutter.dev/go/cascading-menus
The draft Framework PR is here: #109179
The draft Engine PR is here: flutter/engine#31124
This PR is part of the overall effort to support Material 3 - #91605

The current burndown list for implementing the Material menu bar:

• Basic prototype, menus can be defined, open and close.
• Design doc update
• Convert overlay usage into regular Stack
• Figure out how to add all menus offstage initially to allow activator registration.
• Hover support
• Activator support
• Keyboard navigation support
• RTL support
• Visual Density support
• Platform system menu plugin support for macOS
• Platform default menu item support (basic)
• Screen placement and scrolling on small windows.
• Confirm semantics support (with tests) for cascading menus and menubars #127617
• unit tests x1000
• API docs update
• Final formatting pass
• Add examples and example tests
• Connect activators and keyboard support with Shortcuts and Actions?