Flavors are only an officially supported feature for Android, iOS, and (indirectly) MacOS. Running flutter build web --flavor strawberry will result in the tool exiting with this message:

Could not find an option named "flavor".

Run 'flutter -h' (or 'flutter <command> -h') for available flutter commands and options.

However, one can run flutter run -d chrome --flavor strawberry, and the app will run without issue. At best, this is potentially confusing. At worst, it could result in Flutter accidentally supporting flavors for web if/when we add more flavors-related features to the framework.

We should also exit the tool when the user tries to use --flavor with flutter run -d chrome.

I recommend resolving this before implementing #21682