[Feature Request] Add discarded_futures

[ricardodalarme]

Description

Making asynchronous calls in non-async functions usually signals a programming error. These functions should generally be marked async, and such futures should probably be awaited (as enforced by unawaited_futures).

So, to avoid this kind of problem, it would be really nice to have the discarded_futures included in the analysis.

Documentation

• https://dart-lang.github.io/linter/lints/discarded_futures.html

[renancaraujo]

After some reasearch, seems like there are some unresolved false positives issues with this rule.

dart-lang/linter#3801

dart-lang/linter#4085

dart-lang/linter#4375

Dealing with false positives is the biggest blocker for this issue to be added to VG analysis IMO