To date, the Flutter web platform support has been in its own fork. While you can now build an application for the web using Flutter (see here, doing so causes the flutter tool to pull a bunch of cached artifacts that are forked, rather than directly derived from, the mainline.

This can have numerous unforeseen consequences --- impacts to network use and disk space, and because it's a fork, bugs you expect to be fixed in the mainline for a given channel may still not be fixed in the fork.

Flutter for web remains experimental at least until this is resolved, and in all likelihood longer.