Lint request: avoid FutureOr<void>

[dnfield]

Context at #35237

This seems like something that should be easy to lint against.

[srawlins]

If this is a request for a new lint rule to be implemented, that would go to https://github.com/dart-lang/linter. I'm happy to move this issue over if that's the case.

[dnfield]

Ahh yes sorry about that

[srawlins]

They're only 2 letters off 😛 This is surely the first of hundreds of issues to be moved from here to dart-lang/linter.