This reverts commit a46139a2.

* Expose CupertinoModalPopupRoute for state restoration

* Add state restoration test, expose kCupertinoModalBarrierColor

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

A very minimal right-click menu for Mac desktop.

This reverts commit 99daddc2.

  * 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.

* Add 'See also' to cupertino library

* Add snippets to CupertinoApp

* Add CupertinoPageScaffold samples

* Revert "Add CupertinoPageScaffold samples"

This reverts commit 01b0adf351f0320af7835551102d62e3416f0eab.

* Revert "Revert "Add CupertinoPageScaffold samples""

This reverts commit 0af506237a16f7fec5f3744a9bd015a511a15f72.

* Add CupertinoNavigationBar sample

* Tweak CupertinoPageScaffold sample

* Don't use Material in cupertino samples

* Fix colors

* Fix capitalization
Co-authored-by: Shi-Hao Hong <shihaohong@google.com>

* Apply suggestions from code review
Co-authored-by: Shi-Hao Hong <shihaohong@google.com>

* Add comment about backgroundColor
Co-authored-by: Shi-Hao Hong <shihaohong@google.com>