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.

The issue points to packages/flutter_tools/lib/src/vmservice.dart and its Flutter-version service extension. Start by reviewing that extension and how DevTools currently obtains the version for feature experiments; the issue does not define a Dart-version API, tests, or acceptance criteria, so the desired outcome needs clarification first.

Written by the indexing model from the issue text.