title: How to inject Document in service?
body: <p>I have an Angular 2 application. For mocking the  <code>Document</code> object in tests, I'd like to inject it to the service like:</p>
  
  <pre><code>import { Document } from '??' 
  
  @Injectable()
  export class MyService {
    constructor(document: Document) {}
  }
  </code></pre>
  
  <p>The <code>Title</code> service of Angular <a href="https://github.com/angular/angular/blob/master/modules/%40angular/platform-browser/src/browser/title.ts" rel="noreferrer">uses the internal <code>getDOM()</code> method</a>.</p>
  
  <p>Is there any simple way to inject the <code>Document</code> to the service? Also, how should I reference it in the <code>providers</code> array?</p>