[3.6] Add digit separators to Language Tour

[MaryaBelanger]

Do not merge until release -- fixes #6144

[dart-github-bot]

Visit the preview URL for this PR (updated for commit c363979):

https://dart-dev--pr6179-digit-separators-j6jad1gq.web.app

[MaryaBelanger]

I added // ignore_for_file: experiment_not_enabled to pass CI for now, but I guess that didn't do anything. @parlough, when you get a chance, can you remind me the best practice for writing code excerpts for not-yet-released features? (offline works too) Thanks!

Edit: @domesticmouse too if you know the answer (not urgent though)

[domesticmouse]

Personally I prefer to have PRs failing when they aren't targeting current stable. Makes it obvious that they aren't ready to land

[parlough]

Consider adding a version note about the required language version.

[parlough]

I added // ignore_for_file: experiment_not_enabled to pass CI for now, but I guess that didn't do anything. @parlough, when you get a chance, can you remind me the best practice for writing code excerpts for not-yet-released features? (offline works too) Thanks!

There are a few options, including but not limited to:

1. If the current stable version supports the experiment, you can enable the experiment in that project's analysis config.
2. If not or the experiment isn't fully functional and you want to land this, not adding a code excerpt for now is fine (but consider creating a TODO or an issue).
3. If you don't want this to land until the feature is stable, just let the build fail. As Brett mentions that's a good indicator of when support has landed in stable.

In this case, I think this is fine to land now if there's a version note, so the easiest solution might be to just drop the excerpt.