angular2 routes TypeError：无法读取属性’forRoot’

我收到TypeError：无法读取未定义的属性’forRoot'(…)
当我调查时,我发现在执行期间未定义RouterModule.

以下是我的app.route.ts

import {Routes,RouterModule } from '@angular/router';

//const routes: Routes = [];
import {HelloComponent} from './hello.component';
import {IncComponent} from './inc.component';
import {AdditionComponent} from './addition.component';
import {RomanComponent} from './roman.component';

let routes: Routes = [

    {
        path: 'hello',component: HelloComponent
    },{
        path: '',redirectTo: '/hello',pathMatch: 'full'
    },{
        path: 'increment',component: IncComponent
    },{
        path: 'adder',component: AdditionComponent
    },{
        path: 'roman',component: RomanComponent
    }
];


export const routing = RouterModule.forRoot(routes); // error on this line

以下是我的app.module.ts

import { NgModule }      from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';

import { AppComponent }  from './app.component';
import {routing} from './app.routes';
import {HelloComponent} from './hello.component';
import {IncComponent} from './inc.component';
import {AdditionComponent} from './addition.component';
import {RomanComponent} from './roman.component';

@NgModule({
    imports:[
        BrowserModule,routing
    ],declarations: [
        AppComponent,HelloComponent,IncComponent,AdditionComponent,RomanComponent
    ],bootstrap:    [ AppComponent ]
})
export class AppModule { }

和main.ts

import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';
    //import { HTTP_PROVIDERS } from '@angular/http';
    import { AppModule } from './app.module';

    platformBrowserDynamic().bootstrapModule(AppModule);