Add a quick fix for unawaited futures in an async context

[matanlurey]

... i.e. a fix for unawaited_futures.

I'd suggest the following 2 quick fixes to be available:

1. (Preferred) Insert await
2. (Optional) Import package:pendatic (if required), and wrap in unawaited(<expression>).

/cc @davidmorgan @bwilkerson @srawlins

[om-ha]

I think the importance of this the linter rule unawaited_futures is already known by everyone.

Having this quick-fix implemented, augments the usefulness of this linter rule. expression -> unawaited(expression)

For reference, please check original the issue I originally (and incorrectly) posted in Dart-Code #3623

[asashour]

This can be closed, both fixes are now available.