Need a way to specify tab order or position for FxRadioButtonGroup

Steps to reproduce:
1. There is no restriction that FxRadioButtons need to be in the same parent. However, because there is no restriction there is some confusing behavior when tabbing between FxRadioButtonGroups and accessing the FxRadioButtons via the keyboard.

The radio buttons are sorted in a breath-first order so those with a smaller nest level come first. This is confusing because things that are less nested come first, but visually on screen they appears to the user as being at the same nest level

2. To see the behavior, run the attached swf.

Actual Results:
The tab order and the selection order seem to be reversed. In this case tabbing goes from b1->a1, but to get from b1 to b2, you have to hit the up arrow instead of the down arrow.

No way to set index/position in the RadioButtonGroup.

Expected Results:
(From Carol) Be able to set the index/position of the radio button in the group, sort of like you can with children in a DisplayObjectContainer. Or alternatively, have a sort order property on the FxRadioButtonGroup with a couple of choices.

Workaround (if any):