[web] Remove service worker loader and --pwa-strategy - #194785

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One thing I want to flag about dropping FlutterServiceWorkerLoader entirely.

In 3.41–3.47, the loader called getRegistration() and, if a registration existed, re-registered flutter_service_worker.js?v=<new random>. That was an active check with a fresh URL, so it always went to the network and picked up the cleanup stub.

With this PR, apps upgrading from <= 3.38 rely only on the browser's soft update on navigation, which refetches the old URL (flutter_service_worker.js?v=<old>). The browser bypasses its own HTTP cache for that fetch, but a CDN that caches by full URL (long max-age on *.js, no purge on deploy) will keep serving the old caching worker for ?v=<old>. Byte-identical script means no install, so the old worker stays active until the CDN entry expires. That's the exact wedged-app scenario the stub is meant to prevent.

Can we keep a minimal version of the active check in flutter.js? No settings, no public API, and it becomes a no-op once the old worker is gone:

if ('serviceWorker' in navigator) {
  navigator.serviceWorker.getRegistration().then((registration) => {
    const scriptURL = registration?.active?.scriptURL ?? '';
    if (scriptURL.includes('flutter_service_worker.js?v=')) {
      navigator.serviceWorker.register(
        resolveUrlWithSegments(`flutter_service_worker.js?v=${Date.now()}`),
      );
    }
  });
}

registration.update() would be simpler, but it refetches the same ?v=<old> URL so it doesn't help with the CDN case. The ?v= check also keeps us from touching custom service workers that were registered directly.

Also, as far as I can tell the soft-update path was only verified on Chrome. The explicit register() doesn't depend on each browser's update scheduling.

Addressed in b04bc47: narrowed the runtime warnings to ?.serviceWorkerUrl, removed the redundant serviceWorkerVersion = null substitution, added the fire-and-forget cleanupLegacyServiceWorker CDN cache-buster in loader.js, and linked #136620.

Can we keep a minimal version of the active check in flutter.js? No settings, no public API, and it becomes a no-op once the old worker is gone:

Added cleanupLegacyServiceWorker (with Trusted Types support via ttPolicy) in loader.js (b04bc47b4c8) so both loadEntrypoint and load re-register flutter_service_worker.js?v=${Date.now()} when an active registration matches flutter_service_worker.js?v=.

#136620 can be closed now, right?

Yes — added Fixes https://github.com/flutter/flutter/issues/136620 to the PR description so it auto-closes on merge.

Updated compile_web_test.dart for the 3-argument WebBuilder.buildWeb signature after merging master (34c791d); re-requesting review.

autosubmit label was removed for flutter/flutter/194785, because - This pull request has the missing-tests label and does not have the test-exempt label. Please add tests or obtain a test exemption from a tech lead before re-applying the autosubmit label.