Switch to using maybeLocaleOf instead of localeOf - #443

cc @dnfield

Hm. Looks like dart changed some rules. I can push a fix up for it and then merge it into this.

Now coveralls is failing? :\

**The specified coverage format is not supported.** Yeah, that's weird.

I just rebased and pushed. This isn't expected to pass though, until the next dev release, since the requested SDK version doesn't exist yet.

I can switch this to a draft PR until then, if you want.

Yeah let's do that, as long as it doesn't block progress upstream.

Hmm. I tried re-running this now that 1.24.0-10.2.pre is out in dev (and beta), and it fails on Stable. Is that expected? It also still fails coveralls, but that seems like a CI problem.

OK, @dnfield trying again now that the pubspec has advanced the Flutter version.

OK, so maybe I just need to make this change on the nullsafety branch? Seems like the master branch for flutter_svg must be failing for all PRs right now, since this change doesn't modify the pubspec.yaml anymore.

Coveralls is sometimes failing for reasons I don't understand, this change LGTM and should be fine to land as is.