[web] Use Trusted Types in flutter/flutter

Ensure that a Flutter Web app that decides to turn on [Trusted Types API](https://developer.mozilla.org/en-US/docs/Web/API/Trusted_Types_API), can do so.

Depends on: https://github.com/flutter/flutter/issues/112654

Currently, `flutter.js` is setting two unsafe `src` strings (for main.dart.js and flutter_service_worker.js). There may be more offenses.