The Tor Browser / file revision

 // The LHS of a for-loop is not bound to a particular scope until after the .next() method returns.

 var obj = {};

 // Test 1

 function g() {

     obj.x = 0;

     yield 1;

 }

 var x = 2, n = 0;

 with (obj) {

     for (x of g())  // g().next() inserts a binding for x on obj

         n++;

 }

 assertEq(x, 2);

 assertEq(obj.x, 1);

 assertEq(n, 1);

 // Test 2

 function h() {

     delete obj.x;

     yield 3;

 }

 n = 0;

 with (obj) {

     for (x of h())  // h().next() deletes the binding for x on obj

         n++;

 }

 assertEq(x, 3);

 assertEq("x" in obj, false);

 assertEq(n, 1);