Date Picker restructuring. Moved files out of 'pickers' and merged several of the non-public implementation files into date_picker.dart.

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.

[flutter_tools] Make flutter upgrade --verify-only display framework version differences instead of commit hashes (#69420)

Skip flaky iOS XCUITest

This reverts commit ef474154.