Use an unnamed library on which to hang the TestOn annotation

[srawlins]

This is an enforced rule, for packages with an SDK constraint at or above 2.19.

I found this issue on a recent flutter-analyze trybot run for the Dart SDK: https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8785451814245563585/+/u/analyze_flutter_engine/stdout?format=raw

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[srawlins]

CC @yjbanov

[ditman]

@srawlins I didn't see this fix forward so I reverted and relanded my change.

Thanks for the fix though!!