Reland "[linter] Makes `unawaited_futures` report for `FutureOr`"

This reverts commit 5c630788056fba98e715acad97a405718e0f2a3d.

Reason for revert: The issue is still valid, we just found failures in g3.

Original change's description:
> Revert "[linter] Makes `unawaited_futures` report for `FutureOr`"
>
> This reverts commit 6f00c0695c7c2cdddf16776b9a1c272bba70045a.
>
> Reason for revert: Many failures in google3
>
> Failure Link: https://fusion2.corp.google.com/presubmit/947562707/OCL:947562707:BASE:948100983:1784092383598:6c2699f3/targets
>
> Original change's description:
> > [linter] Makes `unawaited_futures` report for `FutureOr`
> >
> > Fixes: https://github.com/dart-lang/sdk/issues/59105
> > Change-Id: I8fcfd5efc2ff6b29cb9cf94353f8e11ea02c8864
> > Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/520882
> > Commit-Queue: Samuel Rawlins <srawlins@google.com>
> > Auto-Submit: FMorschel <git@fmorschel.dev>
> > Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
> > Reviewed-by: Samuel Rawlins <srawlins@google.com>
>
> Change-Id: I56c341fbd650fd152e7c8694bb87d82025a80958
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/524720
> Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
> Auto-Submit: Liam Appelbe <liama@google.com>
> Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
> Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Change-Id: Ib6d78bfc626bb817414d150645f722416504a876
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/525320
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>