router_config_loader.ts

export const ROUTES = new InjectionToken<Route[][]>('ROUTES');

export class RouterConfigLoader {
  // ...
  load(parentInjector: Injector, route: Route): Observable<LoadedRouterConfig> {
    // ...
    const moduleFactory$ = this.loadModuleFactory(route.loadChildren !);
    // ...
  }

  private loadModuleFactory(loadChildren: LoadChildren): Observable<NgModuleFactory<any>> {
    if (typeof loadChildren === 'string') {
      return from(this.loader.load(loadChildren));
    } else {
      return wrapIntoObservable(loadChildren()).pipe(mergeMap((t: any) => {
        if (t instanceof NgModuleFactory) {
          return of (t);
        } else {
          return from(this.compiler.compileModuleAsync(t));
        }
      }));
    }
  }
}