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.

Das Issue verweist auf packages/flutter_tools/lib/src/vmservice.dart und dessen Flutter-version service extension. Beginne damit, diese Extension zu überprüfen und wie DevTools derzeit die Version für Feature-Experimente ermittelt; das Issue definiert keine Dart-version API, Tests oder Akzeptanzkriterien, daher muss das gewünschte Ergebnis zunächst geklärt werden.

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