ServiceWorker: "minimal" end-to-end sample as a W3C test

LayoutTests/http/tests/serviceworker/resources/test-helpers.js

 // Adapter for testharness.js-style tests with Service Workers
 
 function service_worker_test(url, description) {
     var t = async_test(description);
     t.step(function() {
 
         navigator.serviceWorker.register(url).then(
             t.step_func(function(worker) {
                 var messageChannel = new MessageChannel();
                 messageChannel.port1.onmessage = t.step_func(onMessage);
                 worker.postMessage({port:messageChannel.port2}, [messageChannel.port2]);
             }),
             t.step_func(function(reason) {
                 assert_unreached('Registration should succeed, but failed: ' + reason.name);
             }));
 
         function onMessage(e) {
             assert_equals(e.data, 'pass');
             t.done();
         }
     });
 }
+
+// FIXME: Replace this with test.unreached_func(desc) once testharness.js is updated
+// Use with unexpected event handlers or Promise rejection.
+// E.g.:
+//    onbadevent = fail(t, 'Should only see good events');
+//    Promise.then(...).catch(fail(t, 'Rejection is never fun'));
+function unreached_func(test, desc) {
+    return test.step_func(function() {
+        assert_unreached(desc);
+    });
+}