NullInjectorError：没有AngularFirestore的提供者

我正在学习Angular寻找修复错误的帮助：
我正在关注此链接： https://github.com/angular/angularfire2/blob/master/docs/install-and-setup.md
用angular2和angularfirestore2创建一个有角度的小应用程序

但是当我点击服务器时，我在浏览器控制台中收到以下错误..

StaticInjectorError[AngularFirestore]: 
  StaticInjectorError[AngularFirestore]: 
    NullInjectorError: No provider for AngularFirestore!
    at _NullInjector.get (core.js:923)
    at resolveToken (core.js:1211)
    at tryResolveToken (core.js:1153)
    at StaticInjector.get (core.js:1024)
    at resolveToken (core.js:1211)
    at tryResolveToken (core.js:1153)
    at StaticInjector.get (core.js:1024)
    at resolveNgModuleDep (core.js:10585)
    at NgModuleRef_.get (core.js:11806)
    at resolveDep (core.js:12302)

我试过谷歌搜索它，但没有找到确切的解决方案没有对我有用:(，

这是我遵循的：
1)已安装的节点版本8.9.1
2)npm install -g @ angular / cli – >版本1.5.2
3)新的’项目名’
4)npm install angularfire2 firebase –save

这是我的app.module.ts文件：

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { AppComponent } from './app.component';
import { AngularFireModule } from 'angularfire2';
import { environment } from '../environments/environment';




@NgModule({
  imports: [
    BrowserModule,AngularFireModule.initializeApp(environment.firebase)
  ],declarations: [ AppComponent ],bootstrap: [ AppComponent ]
})
export class AppModule {}

app.component.ts：

import { Component } from '@angular/core';
import { AngularFirestore } from 'angularfire2/firestore';
import { Observable } from 'rxjs/Observable';

@Component({
  selector: 'app-root',templateUrl: './app.component.html',styleUrls: ['./app.component.css']
})
export class AppComponent {
  title = 'app';

  constructor(db: AngularFirestore) {

  }
}

environemnt.ts：

export const environment = {
  production: false,firebase: {
    apiKey: 'xxxxx',authDomain: 'aaaaaaa',databaseURL: 'bbbbbbbbbbbbbbbbbb',projectId: 'aaaaaaaaaaaaaa',storageBucket: 'aaaaaaaaaaaa',messagingSenderId: 'aaaaaaaaaaaaa'
  }
};

然后ng服务，我收到上述错误..

@NgModule({
  imports: [
    BrowserModule,providers: [AngularFirestore],bootstrap: [ AppComponent ]
})
export class AppModule {}