--fatal-deprecation excludes obsolete Deprecations

[halo]

Using --fatal-deprecation 1.93.3 causes unrelated warnings:

Warning: mixed-decls deprecation is obsolete, so does not need to be made fatal.
Warning: type-function deprecation is obsolete, so does not need to be made fatal.

I think it is because Depdecation.forVersion does not filter out deprecations that have been marked as obsoleteIn.

This is my attempt to solve it. I hope I don't have to sign the Google Individual Contributor License Agreement, because I don't use a Google Account (I see no way to sign the agreement without an account, though I'm fine with agreeing to its contents, although my code is just a single conditional and the obvious related test). Also, the newly added test passes, and the grinder linted my code (as far as I can tell), but I could not run the -x node tests for some reason.

If this is unacceptable feel free to close this PR. It was a nice exercise learning something about Dart! Thank you for your time.

PS: This project is so well maintained and I'm grateful to @nex3 who kept in in shape for years and put so much effort in documentation and figuring out a path to go forward with the SASS syntax as CSS evolves more and more. Thank you!

Related: #2647

[jathak]

Good catch with this! --fatal-deprecation <version> should definitely not make deprecations that were obsolete as of <version> fatal.

However, I actually think it makes more sense to exclude all deprecations that are obsolete in the currently running version from --fatal-deprecation, even if they weren't yet obsolete in the <version> passed to the flag.

If for instance, --fatal-deprecation 1.91.0 was added to a user's config when 1.91.0 was the current version, it makes sense for the flag to continue to work without warnings in 1.92.0 when mixed-decls and type-function are obsoleted without the user having to bump the fatal version to 1.92.0, which would additionally make with-private fatal.