verticalCenter and horizontalCenter don't work in ConstraintLayout

Steps to reproduce:
1. Create a Group with a ConstraintLayout.
2. Try to use horizontalCenter or verticalCenter to center objects in the columns or rows.

Actual Results:
Nothing happens.

Expected Results:
Centered objects.

Workaround (if any):
Use mx:Canvas.

This is from the forums here: http://forums.adobe.com/message/3890323