Added version note about wildcard variables language feature of Dart 3.7

[ViktorVoloshko]

Dart 3.7 added wildcard variables language feature that prevents _ identifiers from actually declaring a variable. So multiple _ in a single callback now possible.

Added version note about this to according guideline.

---

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[MaryaBelanger]

/gcbrun

[dart-github-bot]

Visit the preview URL for this PR (updated for commit a75b9fb):

https://dart-dev--pr6432-patch-1-kko7bhnz.web.app

[MaryaBelanger]

Thank you for noticing this @ViktorVoloshko, great catch! This is definitely important to update.

@munificent, what do you think about me rewriting the section completely? Or maybe removing it?

For a rewrite, it could a bunch of different ways:

• DON'T use _, __, etc. for unused callback parameters
• DO use wildcard variables for unused callback parameters
• ??

Or, we could leave it as-is with a note like the one in this PR (I'd move it right under the section heading), with a plan to remove/change/update later, when we're further from the latest version release?

[MaryaBelanger]

Thank you @ViktorVoloshko! I'm going to merge this PR. Other stakeholders: I might open another PR to change the title of this section all together; will link to here for reference

[MaryaBelanger]

/gcbrun

[munificent]

Sorry for the delay, I was out all week. I agree that rewriting it eventually would be good. I would probably change the guideline to "PREFER using _ for unused callback parameters".

Then in the prose mention that if targeting Dart < 3.7 that you may need to use __, etc.