Migrate flutter_localizations to null safety.

This reverts commit 1f0df545.

* Remove nullOk from cupertino color resolution

* Add CupertinoDynamicColor.maybeResolveFrom

* Remove CupertinoDynamicColor.maybeResolveFrom again

My yearly pass to fix spelling errors in the API dartdocs.

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.

Enables custom text selection menus by allowing selectionControls to be passed to TextField et. al.

This reverts commit da58f5e6.

* migrate material to nullsafety

* address review comments

* address review comments

* fix build