The Tor Browser / file revision

 var obj;

 var counter = 0;

 var p = Proxy.create({

     has : function(id) {

 	if (id == 'xyz') {

 	    ++counter;

 	    if (counter == 7) {

 		obj.__proto__ = null;

 	    }

 	    return true;

 	}

 	return false;

     },

     get : function(id) {

 	if (id == 'xyz')

 	    return 10;

     }

 });

 function test()

 {

     Object.prototype.__proto__ = null;

     obj = { xyz: 1};

     var n = 0;

     for (var i = 0; i != 100; ++i) {

 	var s = obj.xyz;

 	if (s)

 	    ++n;

 	if (i == 10) {

 	    delete obj.xyz;

 	    Object.prototype.__proto__ = p;

 	}

     }

 }

 try {

     test();

 } catch (e) {}