NullInjectorError：没有 AngularFirestore 的提供者

我正在学习Angular ，以寻求修复错误的帮助: 我正在关注这个链接:https://github.com/angular/angularfire2/blob/master/docs/install-and-setup.md 使用angular2和angularFirestore2创建一个棱角分明的小应用程序

但当我点击ng serve时，我在浏览器控制台中得到以下错误..

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)

我试着在谷歌上搜索，但没有找到确切的解决方案——对我来说什么都不管用:(，

Here is what I followed: 1) Installed Node Version 8.9.1 2) npm install -g @angular/cli --> Version 1.5.2 3) ng new 'project-name' 4) npm install angularfire2 firebase --save

这是我的应用程序.单元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) {}
}

环境.ts:

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

然后发球，我收到了上面的错误.