This PR makes the static API of the `BackgroundIsolateBinaryMessenger` consistent between its web and IO implementations.

This way, multi-platform applications will compile for the web when using the `ensureInitialized` method (even though it'll throw at runtime, like the `instance` getter, because isolates are not supported on the web).

### Issues

* Fixes: https://github.com/flutter/flutter/issues/145260