title: Angular2 Router navigate is not working in service
body: <p>Now I am trying to use Angular2 Router.navigate, but it isn't working.</p>
  
  <pre><code>import { Injectable } from '@angular/core';
  import { Http, Headers } from '@angular/http';
  import { Router } from '@angular/router';
  import { Observable } from 'rxjs/Observable';
  import 'rxjs/Rx';
  
  @Injectable()
  export class HttpService {
  
      constructor(
          public _http: Http,
          public _router: Router
          ) 
      {
      }
  
      public sendPostRequestWithParams(url, params) {
          let headers = new Headers();
          headers.append('Content-Type', 'application/x-www-form-urlencoded');
  
          return this._http.post(url, params, {headers: headers})
          .map(res =&gt; res.json())
          .catch(this.handleServerError);
      }
  
      public handleServerError(error: Response) {
          let jError = error.json() &amp;&amp; error.json()['error'];
          if (jError == 'Unauthenticated.') {
              alert("You are not unauthenticated. Please login correctly!");
              this._router.navigate(['login']);
              return Observable.throw("You are not unauthenticated. Please login correctly!");
          }
          return Observable.throw(error.json() || 'Server error'); // Observable.throw() is undefined at runtime using Webpack
      }
  
  }
  </code></pre>
  
  <p>But when calling handleServerError, it shows error "Cannot read property navigate of undefined".</p>
  
  <p>Is it impossible to use Router in Service?</p>