The Tor Browser / file revision

 // When argument[x] is assigned, where x > callee.length, frame.arguments reflects the change.

 var g = newGlobal();

 g.eval("function f(a, b) {\n" +

        "    for (var i = 0; i < arguments.length; i++)\n" +

        "        arguments[i] = i;\n" +

        "    debugger;\n" +

        "}\n");

 var dbg = Debugger(g);

 var hits = 0;

 dbg.onDebuggerStatement = function (frame) {

     var argc = frame.eval("arguments.length").return;

     var args = frame.arguments;

     assertEq(args.length, argc);

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

         assertEq(args[i], i);

     hits++;

 }

 g.f(9);

 g.f(9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9);

 assertEq(hits, 2);