router_config_loader.ts

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