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.

This reverts commit 7ba775a9.

Move Flutter Gallery to dev/integration_tests/ as it is an older copy used only for testing. The current version of the Flutter Gallery now lives in https://github.com/flutter/gallery.