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

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

Index: LayoutTests/http/tests/serviceworker/resources/test-helpers.js
diff --git a/LayoutTests/http/tests/serviceworker/resources/test-helpers.js b/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
index b863bbb5b08df5a6235e49399dab2c43c4ec0495..c4df888078ec3df7853796bf91ca1b8b14595198 100644
--- a/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
+++ b/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
@@ -20,3 +20,14 @@ function service_worker_test(url, description) {
         }
     });
 }
+
+// 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);
+    });
+}