var yPoint:Number = greenBar.y;

// MouseEvent.MOUSE_MOVE
greenBar.addEventListener(MouseEvent.MOUSE_MOVE, _mouseMove);

function _mouseMove(evt:MouseEvent):void {
    if (evt.currentTarget.y != yPoint) {
        evt.currentTarget.y = yPoint;
    }
}

// dragging
greenBar.addEventListener(MouseEvent.MOUSE_DOWN, _mouseDown);

function _mouseDown(evt:MouseEvent):void {
    evt.currentTarget.startDrag();
    evt.currentTarget.addEventListener(MouseEvent.MOUSE_UP, _mouseUp);
}

// dropping
function _mouseUp(evt:MouseEvent):void
{
    evt.currentTarget.stopDrag();
    evt.currentTarget.removeEventListener(MouseEvent.MOUSE_UP, _mouseUp);
}

stage.addEventListener(MouseEvent.MOUSE_MOVE, _mouseMove);

Error: Error #2071: The Stage class does not implement this property or method.

var drag:Boolean = false;
greenBar.addEventListener(MouseEvent.MOUSE_DOWN, onMouseDown);
greenBar.addEventListener(MouseEvent.MOUSE_UP, onMouseUp);
addEventListener(Event.ENTER_FRAME, updateGreenBar);

function onMouseDown(event:MouseEvent):void
{
    drag = true;
}
function onMouseUp(event:MouseEvent):void
{
    drag = false;
}
function updateGreenBar(event:Event):void
{
    if(drag)
    {
        greenBar.x = mouseX;
    }
}