I'm stuck on document class

I have a preloader on frame 1 of the timeline.

this.addEventListener(Event.ENTER_FRAME, loading);
function loading(event:Event):void {
var total:Number = this.stage.loaderInfo.bytesTotal;
var loaded:Number = this.stage.loaderInfo.bytesLoaded;
if (total == loaded) {
gotoAndPlay("start");
this.removeEventListener(Event.ENTER_FRAME, loading);
}
}
stop();

I also have an example of a document class which I found on the forum from cancerinform(moderator).

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

When I test the movie I get this message:

Discription
1046: Type was not found or was not a compile-time constant: Event.

Source
function loading(event:Event):void {

If I take the document class away then the swf plays fine.
I just need to understand why this is happening.

You need to let the compiler know that you're going to be using the Event class, just like you're using the MovieClip class, add this:

import flash.events.Event;