This reverts commit ff9fe356.

* added Magnifier for iOS and Android

* Support global selection

* addressing comments

* add new test

* Addressing review comments

* update

* addressing comments

* addressing comments

* Addressing comments

* fix build

* Revert "Add default selection style (#100719)"

This reverts commit c8057bc5.

* update

* update

Implements a PlatformMenuBar widget and associated data structures for defining menu bars that use native APIs for rendering.

This PR includes:
A PlatformMenuBar class, which is a widget that menu bar data can be attached to for sending to the platform.
A PlatformMenuDelegate base, which is the type taken by a new WidgetsBinding.platformMenuDelegate.
An implementation of the above in DefaultPlatformMenuDelegate that talks to the built-in "flutter/menu" channel to talk to the built-in platform implementation. The delegate is so that a plugin could override with its own delegate and provide other platforms with native menu support using the same widgets to define the menus.
This is the framework part of the implementation. The engine part will be in flutter/engine#32080 (and flutter/engine#32358)

This reverts commit 988959da.

Simplifying and refactoring parts of RenderEditable. Functionality is the same.

This reverts commit 5f792ba1.

This reverts commit 17870d5d.

Text editing shortcuts involving the arrow keys are no longer handled by RenderEditable's RawKeyboardListener, they use the new Shortcuts setup.  First PR in a plan to port all text editing keyboard handling to shortcuts.

Introduced new widget/ReorderableList and widget/SliverReorderableList widgets.

Very simple right-click context menu for Windows and Linux in a Material-esque style.

  * Making a custom iOS-style text selection menu is now much easier.
  * Exposes a new widget for the toolbar, a new widget for the buttons, and a new widget for the layout.

A new widget that chooses an item from a list based on text input. Just the core widget, with Material and Cupertino versions to come.

* Reland "Nnbd widgets (#64672)"

This reverts commit 127e6790.

* remove non-nullable enforcement on AsyncSnapshot

* make data param non-nullable for AsyncSnapshot.withData

* make some Text params non-nullable