学习到angular2官网上的http的步骤了，出现找不到内存web api

Question

app.module.ts的代码

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';
import { HttpClientModule } from '@angular/common/http';

import { AppComponent } from './app.component';
import { DashboardComponent } from './dashboard/dashboard.component';
import { HeroDetailComponent } from './hero-detail/hero-detail.component';
import { HeroesComponent } from './heroes/heroes.component';
import { HeroService } from './hero.service';
import { MessageService } from './message.service';
import { MessagesComponent } from './messages/messages.component';

import { AppRoutingModule } from './app-routing.module';

import { HttpClientInMemoryWebApiModule } from 'angular-in-memory-web-api';
import { InMemoryDataService } from './in-memory-data.service';

@NgModule({
declarations: [

AppComponent,
HeroesComponent,
HeroDetailComponent,
MessagesComponent,
DashboardComponent

],
imports: [

BrowserModule,
FormsModule,
AppRoutingModule,
HttpClientModule,
HttpClientInMemoryWebApiModule.forRoot(
  InMemoryDataService, { dataEncapsulation: false }
)

],
providers: [

HeroService,
MessageService

], //放服务的地方
bootstrap: [ AppComponent ]
})
export class AppModule { }

in-memory-data.service.ts下的代码