Any application that requires unsound null safety should be forced to pass an explicit flag when running flutter run -d chrome. Auto-detecting unsound null safety should be an error.

It sounds like this is working as intended for all implementations except development mode for the web (JS compiler).

I reproduction case should be to create an app and add a language version comment to the library that contains the entrypoint:

// @dart = 2.11

Then run flutter run -d chrome. It prints a warning but that should be an error.
Running with the explicit flag: flutter run -d chrome --no-sound-null-safety should still work with a warning.