router_module.ts

@NgModule({declarations: ROUTER_DIRECTIVES, exports: ROUTER_DIRECTIVES})
export class RouterModule {
  // ...

  static forRoot(routes: Routes, config?: ExtraOptions): ModuleWithProviders {
    return {
      ngModule: RouterModule,
      providers: [
        // ...
        provideRoutes(routes),
        // ...
      ],
    };
  }

  static forChild(routes: Routes): ModuleWithProviders {
    return {ngModule: RouterModule, providers: [provideRoutes(routes)]};
  }
}

export function provideRoutes(routes: Routes): any {
  return [
    // ...
    {provide: ROUTES, multi: true, useValue: routes},
  ];
}