Fix warnings from integration tests with chromedriver

[jonasfj]

Currently, we have the follow warnings in tests:

Warning: In index.html:53: Local variable for "serviceWorkerVersion" is deprecated. Use "{{flutter_service_worker_version}}" template token instead. See https://docs.flutter.dev/platform-integration/web/initialization for more details.
Warning: In index.html:62: "FlutterLoader.loadEntrypoint" is deprecated. Use "FlutterLoader.load" instead. See https://docs.flutter.dev/platform-integration/web/initialization for more details.

I'm guessing this is something to do with example/web/index.html:
https://github.com/google/webcrypto.dart/blob/master/example/web/index.html

Perhaps we should try to create a new template and copy/cargo-cult the setup from there.

It's unclear if this will fix chromedriver tests on macos.