)

Closes flutter/flutter#160915.

This exact logic was moved to flutter/flutter in flutter/flutter#161249.

…lutter/flutter#161249)

…lutter/flutter#161249)

…lutter/flutter#161249)

…lutter/flutter#161249)

…lutter/flutter#161249)

…61249)

Towards flutter#160915 (future
cleanup: remove the validation from `flutter/cocoon`).

This PR effectively duplicates the logic in
flutter/cocoon#4137, in `flutter/flutter`.

/cc @jtmcdole

…lutter/flutter#161249)

…lutter/flutter#161249)

…lutter/flutter#161249)

…lutter/flutter#161249)

…lutter/flutter#161249)

issue: flutter#188640

This follows the pattern from
flutter#161249

This duplicates the logic of
https://github.com/flutter/cocoon/blob/main/app_dart/bin/validate_task_ownership.dart

Note that the TESTOWNERS file format seems straightforward but it is
actually weird where things that look like comments have semantic
meaning.

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