The Tor Browser / file revision

 // for-of is defined in terms of basic operations on objects, particularly

 // [[Get]] for properties named "iterator" and "next", and [[Call]]. These

 // "semantics" tests check that for-of really does appear to be implemented in

 // terms of those more basic operations, as required by the spec, even in

 // unusual cases.

 // Deleting Array.prototype.iterator makes for-of stop working on arrays.

 load(libdir + "asserts.js");

 load(libdir + "iteration.js");

 delete Array.prototype[std_iterator];

 assertThrowsInstanceOf(function () { for (var x of []) ; }, TypeError);