Bring up to analyzer 8.1.1, get rid of deprecated classes and methods.

[fshcheglov]

Also deleted

    if (element case final Annotatable annotatable) {
      final annotations = annotatable.metadata2.annotations;
      if (annotations.isEmpty) {
        return null;
      }
    }

from source_gen/lib/src/type_checker.dart because it repeated a check that was done a few lines later.

[davidmorgan]

Thanks! Also, hi, Konstantin mentioned you have started contributing to Dart :)

I'm not sure if we want to drop support for analyzer 7 quite yet, but there should not be many code changes so we can use this when we do.

The reason I am not sure is that for #775 we want to do a breaking change / major version release of source_gen: remove all mirrors use then bump to 4.0.0. It might make sense to do that before dropping support for analyzer 7. It's an easy change so maybe we should just do that ... on the other hand maybe we can think of something else we should do before either of the two :)

@kevmoo any thoughts?