private function rotateAroundInternalPoint(m:Matrix, x:Number, y:Number, angleDegrees:Number):void {
	var p:Point = m.transformPoint(new Point(x, y));
	rotateAroundExternalPoint(m, p.x, p.y, angleDegrees);
}

private function rotateAroundExternalPoint(m:Matrix, x:Number, y:Number, angleDegrees:Number):void {
	m.translate(-x, -y);
	m.rotate(angleDegrees * (Math.PI / 180));
	m.translate(x, y);
}

http://www.joelconnett.com/flex-rotation-around-a-point.html

Rotation around point

ActionScript 3