There is news on this topic:
http://esotericsoftware.com/forum/Will-Spine-Runtime-support-for-Flutter-16672?p=73304

And as I see it, many people are interested in this topiс

You are correct, we have indeed noticed the interest in using Flutter with the Spine Runtimes.
Nate has added the Flutter thread to our Spine Runtimes page, with a link to the Github page for the project as well: Spine: Runtimes: Flutter

Luke ：

You are correct, we have indeed noticed the interest in using Flutter with the Spine Runtimes.

The library https://github.com/jtakakura/spine_core only supports version 3.8. It would be nice to see your team involved in official support for this library.

Luke ：

Nate has added the Flutter thread to our Spine Runtimes page, with a link to the Github page for the project as well: Spine: Runtimes: Flutter

I don't think we can split Dart & Flutter for animation as it is now on the Spine Runtime page. It's worth bundling them together.

The library https://github.com/jtakakura/spine_core only supports version 3.8. It would be nice to see your team involved in official support for this library.

Please see Mario's initial reply to the post about Flutter. It is on our radar, but we're currently working hard on improving the Spine Editor and adding features like physics.

I don't think we can split Dart & Flutter for animation as it is now on the Spine Runtime page. It's worth bundling them together.

Thank you for the suggestion; please note that this third party runtime has been included in both the section for Dart and for Flutter itself.