Add script to remove experiments from tests when they have launched.

[lrhn]

Re: #1779 (comment)

I made a script to remove experiments from tests, and update the min-SDK version.

I haven't figured out how to find the matching analyzer package and update that in pubspec as well.
May need to run dart pub upgrade after running this script, or manually updating the analyzer package
to a version that supports the new language version without experiment flags.

[lrhn]

(Will format and remove unused members on Monday.)

[lrhn]

Turns out the unreachable_from_main lint has false positives, so ignored for now.

[lrhn]

I think this can be landed.
Also Dart 3.10 has been released, so the result of running the script (#1779) can also be landed. I'll make sure it's up-to-date.

PTAL.

[lrhn]

Ping @munificent

[lrhn]

(Have we considered moving this package into the Dart SDK repo, so we can more easily keep them in sync?)

[munificent]

This is really cool! But I think it also re-invents some stuff that would be easier to reuse: arg parsing, test file parsing, and version comparison.

[lrhn]

Uses args, pub_semver and TestFile now.

I needed to add output-functionality to TestFile (I did not want to make my own localized writing logic that had to be kept up-to-date, so it's put into the classes now.)
Maybe you can use that in update_tests.dart too.

PTAL.