How to turn off layer visibility ?

<?xml version="1.0" encoding="utf-8"?>
<mx:Application
  xmlns:mx="http://www.adobe.com/2006/mxml"
  xmlns:framework="com.esri.aws.osgi.framework.*"
  xmlns:map="com.esri.aws.awx.map.*"
  xmlns:layers="com.esri.aws.awx.map.layers.*"
  layout="absolute"
  >
  <mx:Script>
    <![CDATA[
      import com.esri.aws.awx.map.layers.LayerVisibility;
      private function onClick() : void
      {
        world.layerVisibilities = [
          new LayerVisibility("Major Rivers", false)
        ];        
      }
    ]]>
  </mx:Script>
  <framework:Framework apiKey="19640523"/>
  <mx:Panel width="100%" height="100%">
    <map:Map>
      <map:basemaps>
        <layers:ArcIMSGroupLayer id="world"
          serviceHost="http://www.geographynetwork.com"
          serviceName="ESRI_World"
          autoLoadLayerVisibilities="true"
          />
      </map:basemaps>
    </map:Map>
    <mx:ControlBar>
      <mx:Button label="Turn Off Major Rivers" click="onClick()"/>
    </mx:ControlBar>
  </mx:Panel>
</mx:Application>