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.

* Migrate more tests

* fix

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* fix navigator observer announcement order

* addressing review comments

* update

* fix analyzer

This reverts commit aee9e94c.

This reverts commit 9a6eb7de.

Fix tests that rely on FadeTransition incorrectly and depend on implementation details of FadeUpwardsTransitionBuilder (#51150)

Navigator 2.0: Refactor the imperative api to continue working in the new navigation system (#44930)