Fixes future warning for awaiting Future returns in async bodies inside try blocks

[FMorschel]

This is merely an analyzer change to avoid the bug explained at:

• Linter rule to prevent returning futures from the try block dart-lang/sdk#62555.

This will allow the development of a warning about these situations so we can avoid this bug in the future.

I'm not sure my changes fall into this category:

Please add a note to packages/devtools_app/release_notes/NEXT_RELEASE_NOTES.md if your change requires release notes. Otherwise, add the 'release-notes-not-required' label to the PR.

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• I listed at least one issue that this PR fixes in the description above.
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• I added new tests to check the change I am making, or there is a reason for not adding tests.