The Tor Browser / file revision

 // Basic for-of test with Proxy whose iterator method is a generator.

 var arr = ['a', 'b', 'c', 'd'];

 var proxy = Proxy.create({

     getPropertyDescriptor: function (name) {

         if (name == 'iterator') {

             return {

                 configurable: false,

                 enumerable: false,

                 writeable: false,

                 value:  function () {

                     for (var i = 0; i < arr.length; i++)

                         yield arr[i];

                 }

             };

         }

         // Otherwise, inherit the property from arr.

         for (var obj = arr; obj; obj = Object.getPrototypeOf(obj)) {

             var desc = Object.getOwnPropertyDescriptor(obj, name);

             if (desc)

                 return desc;

         }

         return undefined;

     }

 });

 for (var i = 0; i < 2; i++)

     assertEq([v for (v of proxy)].join(","), "a,b,c,d");