package
{
	import flash.display.MovieClip;
	public class Main extends MovieClip
	{
		public function Main()
		{

		}
	}
}

http://active.tutsplus.com/tutorials/actionscript/quick-tip-how-to-use-a-document-class-in-flash/

Using class files sets your code free. And it’s really no harder than coding on the timeline; it just involves a little more setup.

Basic empty Document Class

class

ActionScript 3