给弹出框加ICON

<?xml version="1.0" encoding="utf-8"?> 
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
        layout="vertical" 
        verticalAlign="middle" 
        backgroundColor="white"> 
  
    <mx:Script> 
        <![CDATA[ 
            import mx.controls.Alert; 
  
            [Embed(source="assets/bulletCritical.png")] 
            private var BulletCritical:Class; 
  
            [Embed(source="assets/iconCritical.png")] 
            private var IconCritical:Class; 
  
            private var alert:Alert; 
  
            private function init():void { 
                var alertText:String = "Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Suspendisse rutrum metus nonummy augue. In hac habitasse platea dictumst. Nulla arcu libero, nonummy non, suscipit a, mollis non, augue. Maecenas porttitor urna vel enim. Nam eget tortor. Mauris facilisis suscipit felis. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Proin nunc turpis, venenatis non, laoreet at, fringilla nec, purus. Sed sodales. Sed turpis. Vestibulum sagittis justo id metus. Sed placerat, nibh lobortis mattis adipiscing, sapien wisi interdum arcu, nec vehicula sem tortor id nibh."; 
                var alertTitle:String = "The quick brown fox jumped over the lazy dog."; 
                alert = Alert.show(alertText, alertTitle, 4, null, null, IconCritical); 
                alert.titleIcon = BulletCritical; 
            } 
        ]]> 
    </mx:Script> 
  
    <mx:Button label="Launch alert" click="init();" /> 
  
</mx:Application>