Registering the service worker immediately after the documented has loaded may cause SW initialization to compete with framework initialization. It was recommended to us that we defer the service worker setup until after the framework is done with setup, which should be sometime after the first frame.
To implement this, I augmented the binding setup to dispatch an event on the document after the binding has initialized. I don't see any obvious risks with this setup.
Fixes#66066
Move Flutter Gallery to dev/integration_tests/ as it is an older copy used only for testing. The current version of the Flutter Gallery now lives in https://github.com/flutter/gallery.
