Hi,
Thanks for the question. StageXL is built on top auf Canvas2D and WebGL for the web, so there is an abstraction layer to implement the display tree, the animations and many other things on top of different low level render APIs. So someone would need to implement it on top of the render API of Flutter. Unfortunately i am involved in very different projects lately, mostly on the backend, and therefore i do not plan to do this implementation. But it is absolutely possible and i think it is not even that difficult. It would be great to see StageXL on Flutter!
Hope this helps a little bit,
Bernhard