The Tor Browser / file revision

 // defineProperty can define accessor properties.

 var g = newGlobal();

 var dbg = new Debugger;

 var gw = dbg.addDebuggee(g);

 g.value = undefined;

 g.eval("function gf() { return 12; }\n" +

        "function sf(v) { value = v; }\n");

 var gfw = gw.getOwnPropertyDescriptor("gf").value;

 var sfw = gw.getOwnPropertyDescriptor("sf").value;

 gw.defineProperty("x", {configurable: true, get: gfw, set: sfw});

 assertEq(g.x, 12);

 g.x = 'ok';

 assertEq(g.value, 'ok');

 var desc = g.Object.getOwnPropertyDescriptor(g, "x");

 assertEq(desc.configurable, true);

 assertEq(desc.enumerable, false);

 assertEq(desc.get, g.gf);

 assertEq(desc.set, g.sf);