private var _hash:Object = {};

private function onKeyReleased(event:KeyboardEvent):void {
    delete _hash[event.keyCode];
}

private function onKeyPressed(event:KeyboardEvent):void {
    _hash[event.keyCode] = 1;
}

private function isKeyDown(code:int):Boolean {
    return _hash[code] !== undefined;
}

stage.addEventListener(KeyboardEvent.KEY_UP, onKeyReleased);
stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyPressed);