We have some tests that use @TestOn('browser') (or 'chrome', which maybe is a bug?), but in CI we only run Dart unit tests in a browser if they are part of a web plugin implementation. That means we're missing some test coverage for, e.g., cross_file.

We should scan for @TestOn in unit test folders, and if we find any, ensure that we are running tests in the appropriate mode. I believe that could just replace our current plugin-based logic, but we'll need to verify that.