Because of a small bug in the Flex SDK we have to define an empty Group at the 1st position for now.

ActionBarSkin.mxml

<?xml version="1.0" encoding="utf-8"?>
<s:Skin xmlns:fx="http://ns.adobe.com/mxml/2009" 
		xmlns:s="library://ns.adobe.com/flex/spark" xmlns:fxg="skin.fxg.*">
	<!-- host component -->
	<fx:Metadata>
		[HostComponent("spark.components.ActionBar")]
	</fx:Metadata>
	
	<s:states>
		<s:State name="normal" />
	</s:states>
	
	<s:Rect>
		<s:fill>
			<s:LinearGradient rotation="90">
				<s:GradientEntry color="0x71a8ce" />
				<s:GradientEntry color="0x4f83aa" />
			</s:LinearGradient>
		</s:fill>
	</s:Rect>
	
	<s:Group />
	<s:Group id="navigationGroup" left="0" height="50"  />
	<s:Group id="titleGroup" horizontalCenter="0" height="50" />
	<s:Group id="actionGroup" right="0" height="50" />
	
	<s:Label id="titleDisplay" verticalCenter="0" horizontalCenter="0" />
	
</s:Skin>