Fix per bzbarsky's review

In whatwg/html#1995 (comment)

workers/Worker_ErrorEvent_after_navigation.htm

@@ -21,10 +21,10 @@
   const windowProxy = iframe.contentWindow;
 
   window.addEventListener("load", t.step_func(() => {
+    windowProxy.onerror = t.unreached_func("The navigated-away-from Window error event must never fire");
     windowProxy.doWorkerStuffAndExpectNoError(t);
 
     iframe.addEventListener("load", t.step_func(() => {
-      windowProxy.onerror = t.unreached_func("The navigated-away-from Window error event must never fire");
 
       // The test passes if no error events have been fired within this time.
       setTimeout(t.step_func_done(), 300);

workers/Worker_ErrorEvent_after_navigation_support-1.htm

@@ -15,10 +15,7 @@
 window.doWorkerStuffAndExpectError = t => {
   const worker = new Worker("./support/error-after-some-time.js");
   worker.onerror = t.step_func_done(ev => {
-    // Cannot use assert_equals since including testharness.js makes it believe this is a test file.
-    if (ev.constructor !== ErrorEvent) {
-      throw new Error("The worker error event must fire");
-    }
+    parent.assert_equals(ev.constructor, ErrorEvent, "The worker error event must fire");
   });
 
   worker.postMessage("error expected");