refactor(core): private export token to disable `fixture.detectChange…

…s` error

While we do want to discourage `fixture.detectChanges`, it is yet to be
determined how this should be accomplished (warning, documentation, etc.). At
the moment, not being able to disable the error at all is entirely
prohibitive for existing test suites to use zoneless change detection in
tests.

packages/core/testing/private_export.ts

@@ -0,0 +1,9 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+export {AllowDetectChangesAndAcknowledgeItCanHideApplicationBugs as ɵAllowDetectChangesAndAcknowledgeItCanHideApplicationBugs} from './src/test_bed_common';

packages/core/testing/src/fake_async.ts

@@ -26,10 +26,9 @@ export function resetFakeAsyncZone(): void {
 }
 
 export function resetFakeAsyncZoneIfExists(): void {
-  if (!fakeAsyncTestModule) {
-    return;
+  if (fakeAsyncTestModule) {
+    fakeAsyncTestModule.resetFakeAsyncZone();
   }
-  fakeAsyncTestModule.resetFakeAsyncZone();
 }
 
 /**