mc=createEmptyMovieClip('mc',1);

mc.__proto__.addProperty("val",function(){ },function(x){
										_root.doSomething(x);});
//追加
trace(MovieClip.prototype === mc.__proto__);

mc.loadMovie('b.swf');

_root.doSomething = function(v){
	trace('doSomething: '+ v);
	delete(mc.__proto__.val);
        //これも要りそう
       mc.val = v;
}

//※追加
val = '???';

id = setInterval(function(){
				  val = 'xxx';clearInterval(id);},1000);

id2 = setInterval(function(){
				 trace("value: " + val);clearInterval(id2);},2000);