Dart VM != Dart server in this case.
The focus of Dart 2 and beyond is the client side [0].
You can definitely run Dart in the VM, but beyond the HttpServer API, server-side support is mostly a community effort.
My point isn't that this is a bad thing, it's that it makes sense for Firebase to be promoted because Dart doesn't have _official_ server-side support.
[0]: https://medium.com/dartlang/announcing-dart-2-80ba01f43b6