fix(platform-server): wait for async app initializers to complete before removing server side styles (#16712)

This fixes a flicker when transitioning from server rendered page to client rendered page in lazy loaded routes by waiting for the lazy loaded route to finish loading, assuming initialNavigation on the route is set to 'enabled'.

Fixes #15716

packages/core/test/application_init_spec.ts

@@ -5,6 +5,7 @@
  * 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
  */
+import {Injector} from '@angular/core';
 import {APP_INITIALIZER, ApplicationInitStatus} from '../src/application_init';
 import {TestBed, async, inject} from '../testing';
 
@@ -14,11 +15,13 @@ export function main() {
 
       it('should return true for `done`',
          async(inject([ApplicationInitStatus], (status: ApplicationInitStatus) => {
+           status.runInitializers();
            expect(status.done).toBe(true);
          })));
 
       it('should return a promise that resolves immediately for `donePromise`',
          async(inject([ApplicationInitStatus], (status: ApplicationInitStatus) => {
+           status.runInitializers();
            status.donePromise.then(() => { expect(status.done).toBe(true); });
          })));
     });
@@ -26,15 +29,32 @@ export function main() {
     describe('with async initializers', () => {
       let resolve: (result: any) => void;
       let promise: Promise<any>;
+      let completerResolver = false;
       beforeEach(() => {
+        let initializerFactory = (injector: Injector) => {
+          return () => {
+            const initStatus = injector.get(ApplicationInitStatus);
+            initStatus.donePromise.then(() => { expect(completerResolver).toBe(true); });
+          }
+        };
         promise = new Promise((res) => { resolve = res; });
-        TestBed.configureTestingModule(
-            {providers: [{provide: APP_INITIALIZER, multi: true, useValue: () => promise}]});
+        TestBed.configureTestingModule({
+          providers: [
+            {provide: APP_INITIALIZER, multi: true, useValue: () => promise},
+            {
+              provide: APP_INITIALIZER,
+              multi: true,
+              useFactory: initializerFactory,
+              deps: [Injector]
+            },
+          ]
+        });
       });
 
       it('should update the status once all async initializers are done',
          async(inject([ApplicationInitStatus], (status: ApplicationInitStatus) => {
-           let completerResolver = false;
+           status.runInitializers();
+
            setTimeout(() => {
              completerResolver = true;
              resolve(null);