实现Angular Universal，我的app-root不包含任何标签

<app-root></app-root>

npm install @angular/platform-server @nguniversal/module-map-ngfactory-loader ts-loader --save

import { NgModule } from '@angular/core';
import { ServerModule } from '@angular/platform-server';
import { ModuleMapLoaderModule } from '@nguniversal/module-map-ngfactory-loader';
import { AppModule } from './app.module';
import { AppComponent } from './app.component';

@NgModule({
  imports: [
    AppModule,
    ServerModule,
    ModuleMapLoaderModule
  ],
  bootstrap: [AppComponent]
})
export class AppServerModule { }

import { enableProdMode } from '@angular/core';
import { platformServer } from '@angular/platform-server';
import { AppServerModule } from './app/app.server.module';

enableProdMode();

export default platformServer().bootstrapModule(AppServerModule);

import { enableProdMode } from '@angular/core';
import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';
import { AppModule } from './app/app.module';
import { environment } from './environments/environment';

if (environment.production) {
  enableProdMode();
}

platformBrowserDynamic().bootstrapModule(AppModule)
  .catch(err => console.error(err));

"scripts": {
  "build:ssr": "npm run build:client-and-server-bundles && npm run webpack:server",
  "serve:ssr": "node dist/server",
  "build:client-and-server-bundles": "ng build --prod && ng run your-app-name:server",
  "webpack:server": "webpack --config webpack.server.config.js --progress --colors"
}

npm run build:ssr
npm run serve:ssr