Hi,
I think this feature was removed quite some time ago. You can replace it with two options, depending on your use case.
You can use the Mask.relativeToParent flag which indicates if the Mask is relative to the display object where it is applied to, or relative to the parent of this DisplayObject.
Or you can use the Mask.transformationMatrix where you can set a 2D transformation Matrix. You can calculate this matrix on your own, or use the helper method of DisplayObject which is called getTransformationMatrix.
Hope this helps!
Bernhard