The Tor Browser / file revision

 // Observably different visits to the same with-statement produce distinct Environments.

 var g = newGlobal();

 g.eval("function f(a, obj) { with (obj) return function () { return a; }; }");

 var dbg = Debugger(g);

 var hits = 0;

 dbg.onDebuggerStatement = function (frame) {

     // Even though the two visits to the with-statement have the same target

     // object, Math, the environments are observably different.

     var f1 = frame.eval("f(1, Math);").return;

     var f2 = frame.eval("f(2, Math);").return;

     assertEq(f1.environment !== f2.environment, true);

     assertEq(f1.object, f2.object);

     assertEq(f1.call().return, 1);

     assertEq(f2.call().return, 2);

     hits++;

 };

 g.eval("debugger;");

 assertEq(hits, 1);