The Tor Browser / file revision

 // Source.prototype.element can be an object or undefined.

 var g = newGlobal();

 var dbg = new Debugger;

 var gw = dbg.addDebuggee(g);

 g.evaluate("function f(x) { return 2*x; }", {element: { foo: "bar" }});

 var fw = gw.getOwnPropertyDescriptor('f').value;

 assertEq(typeof fw.script.source.element, "object");

 assertEq(fw.script.source.element instanceof Debugger.Object, true);

 assertEq(fw.script.source.element.getOwnPropertyDescriptor("foo").value, "bar");

 g.evaluate("function f(x) { return 2*x; }");

 var fw = gw.getOwnPropertyDescriptor('f').value;

 assertEq(typeof fw.script.source.element, "undefined");