The Tor Browser / file revision

 // for-of on a proxy causes a predictable sequence of trap calls.

 load(libdir + "iteration.js");

 var s = '';

 var i = 0;

 var next_fn = Proxy.createFunction({}, function () {

     s += "n";

     if (i == 3)

         return { value: undefined, done: true };

     return { value: i++, done: false };

 });

 var it = Proxy.create({

     get: function (receiver, name) {

         if (name == 'toSource') {

             s += '?';

             return function () 'it';

         }

         assertEq(name, "next");

         s += "N";

         return next_fn;

     }

 });

 var iterator_fn = Proxy.createFunction({}, function () {

     s += 'i';

     return it;

 });

 var obj = Proxy.create({

     get: function (receiver, name) {

         assertEq(name, std_iterator);

         s += "I";

         return iterator_fn;

     }

 });

 for (var v of obj)

     s += v;

 assertEq(s, 'IiNn0Nn1Nn2Nn');