harness_browser is I believe the chrome app test harness? This file shouldn't be a part of that library as windows is only available to extensions.

We'll need to figure our a good story for testing the extensions API. We may be able to create an extension and use it as a test harness, similar to the chrome one. A main page would load the extension, and run through most (all?) of the unit tests. Definitely work for future CLs though. Does this sounds reasonable, or do you have an existing way to run the test_windows.dart tests?