This reverts commit d37b8621, relanding #22267, now that flutter/flutter#70252 has landed to prevent the problems it had the first time.

Here is the original PR description:

For some reason (probably a bad merge on my part), the return values for locale and locales are not the same on web as they are on other platforms for the PlatformDispatcher and SingletonFlutterWindow classes. The web versions are non-nullable and the others are nullable.

This converts them to be the same (non-nullable) values