gecko-dev/netwerk/test/unit/test_bug935499.js

function run_test() {
  var idnService = Cc["@mozilla.org/network/idn-service;1"]
                   .getService(Ci.nsIIDNService);

  var isASCII = {};
  Assert.equal(idnService.convertToDisplayIDN("xn--", isASCII), "xn--");
}