title: Inject provider into another provider
body: <p>Let's say we have a Component called <code>Comp</code> and two <code>@Injectable</code> providers called <code>P1</code> and <code>P2</code>.</p>
  
  <p><code>P1</code> needs an instance of <code>P2</code>. <code>P1</code> is injected into <code>Comp</code>.</p>
  
  <p>It's working perfectly if I declare both providers on <code>Comp</code> like this:</p>
  
  <pre><code>@Component ({
      providers: [P1, P2]
  })
  export class Comp { ... }
  </code></pre>
  
  <p>Now, what I would like to do is to declare that <code>P1</code> needs <code>P2</code> directly inside <code>P1</code>:</p>
  
  <pre><code>@Component ({
      providers: [P1]
  })
  export class Comp { ... }
  
  
  @Injectable(/** Inject P2 here **/)
  export class P1 { ... }
  </code></pre>
  
  <p>How to achieve this?</p>