package {
	import flash.display.Sprite;
	import flash.text.Font;

	[SWF(width='600', height='400', frameRate='30', backgroundColor='#000000')]

	public class App extends Sprite {
		[Embed(systemFont='Arial',fontName='Arial',mimeType='application/x-font')]
		private var arialFont:Class;

		public function App() {
			init();
		}

		private function init():void {
			Font.registerFont( arialFont );
		}
	}
}

AS3 Embed a system font in Flex

Flex

ActionScript 3