Consider `unawaited_futures` for inclusion

[mit-mit]

We had initially selected unawaited_futures for inclusion in core lints, however ran into some issues that need resolving first, including:

• method-side opt-out for unawaited_futures sdk#58348, Method-side opt-out for unawaited_futures
• Move unawaited to package:meta? sdk#58351, Move unawaited to package:meta
• Enable the lint in the flutter codebase for dogfooding (opting out some Navigator and AnimationController methods, maybe others)

[jacob314]

+1 on adding a method side opt out for unawaited_futures. We see a very large # of calls to unawaited in g3 (about 10% of all calls to awaited). Likely a small # of method side opt outs can reduce this # significantly.

[pq]

@goderbauer: do any Flutter APIs spring to mind that you'd want to tag as unwaitable? it'd be handy to have some concrete examples to drive the discussion.

[goderbauer]

The Future-returning APIs on AnimationController and Navigator are the most common examples, things like AnimationController.forward() / AnimationController.reverse() and Navigator.push() as well as its similarly named friends.

[pq]

Corresponding issue to discussion an annotation in pkg:meta here: dart-lang/sdk#46218.

[natebosch]

https://api.dart.dev/stable/2.14.4/dart-async/unawaited.html

[mit-mit]

@pq to create a proposal for how this would work