* Create state restoration templates, 

* Update existing state restoration samples to use the new templates

* Add to README, decided to require stateful widgets to include RestorationMixin

Update samples to use repo analysis options, Fix sample templates and a ton of analyzer issues (#77868)

This reverts commit 705cebb2.

* Alert dialog dartpad demo

* Updated description

Adds Navigator.maybeOf to replace calling Navigator.of(context, nullOk: true), and removes the nullOk parameter. Also changes Navigator.of to return a non-nullable value, and removes many (120!) instances of the ! operator, reducing the possible places where a null dereference could occur.

Adds MediaQuery.maybeOf to replace calling MediaQuery.of(context, nullOk: true), and removes the nullOk parameter. Also changes MediaQuery.of to return a non-nullable value, and removes many instances of the ! operator, reducing the possible places where a null dereference could occur.

* fix simple dialog introducing additional node for semantics label

* add test

* migrate material to nullsafety

* address review comments

* address review comments

* fix build

* Fixes typos in showDialog documentation

This reverts commit fd0554ba.

Updated references to the old button classes to the new ones in comments and documentation for classes in the material library. (#62092)