title: .filter is not a function Angular2
body: <p>i getting an error <code>EXCEPTION: TypeError: articles.filter is not a function</code>
  in my service </p>
  
  <pre><code>import {Injectable} from 'angular2/core';
  import {Http} from 'angular2/http';
  import 'rxjs/Rx';
  
   @Injectable()
  
   export class ArticlesService {
       http:Http;
       constructor(http:Http) {
       this.http = http;
      }
  
  getArticle(id: number) {
  return Promise.resolve('./articles.json').then(
    articles =&gt; articles.filter(article =&gt; article.id === id)[0]
       );
    }
   }
  </code></pre>
  
  <p><a href="http://plnkr.co/edit/iFYtlbDte83umKgiUcTl?p=preview" rel="nofollow">My Plunker</a> but in "Tour of Heroes" it works fine</p>