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Oh, wait, I missed your comment:

Tests: couldn't test this, because AFAIK we don't have the ability to swap a Flutter Web service worker on the fly in our testing environment :/

What causes the service worker to change to trigger this code path? This test is attempting to do something like that:

https://github.com/flutter/flutter/blob/master/dev/bots/service_worker_test.dart

What causes the service worker to change to trigger this code path? This test is attempting to do something like that:

I think what we need to do is to have:

• An app running with a given serviceWorkerVersion, so it registers that serviceWorker (v1)
• Rebuild the app with a different serviceWorkerVersion (v2)
• Refresh the app. That should trigger the "serviceWorkerVersion is different, I need to update the SW" branch of the code

(I think, not entirely sure)

I've added a new test to service_worker_test.dart that implements what I described above:

1. Load app v1
2. Reload app v1
3. Change app to v2
4. Load app v2

That does exercise the code path where the serviceworker IS updated, but it doesn't trigger the issue on my machine :S

Also expanded the service worker test to have 3 different initializations: without flutter js, with flutter js and all the steps, and with flutterJS "short" (calling autoStart directly).