Restrict feature experiments based on Dart version instead of Flutter version

As part of tackling https://github.com/flutter/devtools/issues/9438, I discovered that `flutter_tools` registers a service extension that we can use to fetch the current Flutter version (see https://github.com/flutter/flutter/blob/4d7ff0982c0f17a361268b5b3b5e1d9688d7db34/packages/flutter_tools/lib/src/vmservice.dart#L222) but there is no service extension equivalent that we can call for a pure Dart app.

I think this is fine for now, but in the future we might want to restrict features based on the Dart version and not the Flutter version.