The Tor Browser / file revision

 // Deleting the .next method of an iterator in the middle of a for-of loop

 // causes a TypeError at the next iteration.

 load(libdir + "asserts.js");

 load(libdir + "iteration.js");

 var iterProto = Object.getPrototypeOf([][std_iterator]());

 var s = '';

 assertThrowsInstanceOf(function () {

     for (var v of ['duck', 'duck', 'duck', 'goose', 'FAIL']) {

         s += v;

         if (v === 'goose')

             delete iterProto.next;

         s += '.';

     }

 }, TypeError);

 assertEq(s, 'duck.duck.duck.goose.');