The Tor Browser / file revision

 function run_test() {

   var Cu = Components.utils;

   sb = new Cu.Sandbox('http://www.example.com',

                       { wantGlobalProperties: ["TextDecoder", "TextEncoder"] });

   sb.do_check_eq = do_check_eq;

   Cu.evalInSandbox('do_check_eq(new TextDecoder().encoding, "utf-8");' +

                    'do_check_eq(new TextEncoder().encoding, "utf-8");',

                    sb);

   Cu.importGlobalProperties(["TextDecoder", "TextEncoder"]);

   do_check_eq(new TextDecoder().encoding, "utf-8");

   do_check_eq(new TextEncoder().encoding, "utf-8");

 }