You can do it with the classic Flash API quite easily. You define a radial fill method, then draw something with it. The key class is GraphicsGradientFill. If you specify the
GradientType.RADIAL it does a radial fill, much like you can do in a graphics app. The rest of the parameters are interpreted accordingly.
A long time since I used it but the only way I found to work with it was set it up and play around with it until I got something I liked. Performance wise it's fast for the classic API but you have the overhead of uploading it to a Texture, which could add up if you were doing it all the time.
If you really want to avoid the classic API there's nothing similar built into Stage3D. The simplest way would be to use a custom MeshEffect, which would have excellent performance but would involve AGAL shader programming.