Flex动态改变按钮图标(icon)

由于Flex支持在AS中动态修改icon属性，所以用clearStyle和setStyle方法

定义图标
[Embed (source="assets/assets.swf", symbol="left_arrow")]
[Bindable]
private static var DEFAULT_ICON:Class;

在AS中动态绑定
(event.target as Button).clearStyle("icon");
(event.target as Button).setStyle("icon", DEFAULT_ICON);