Hi,
I'd like to let you know that I've succesfully implemented a Dart Spine runtime (based on the libgdx runtime):
GitHub: https://github.com/FedeOmoto/spine/
Pub: http://pub.dartlang.org/packages/spine
Here you can find a few demos using the runtime:
http://fedeomoto.github.io/spine/
Best regards,
Federico Omoto