title: RxJS Array of urls to GET requests merged
body: <p>I'm brand new to RxJS and want to do the following (in Angular 2):</p>
  
  <ol>
  <li>Request some JSON from the server containing an array of urls</li>
  <li>Make a request to each of those urls</li>
  <li>Subscribe to the combined completion of all of those requests</li>
  </ol>
  
  <p>At the moment my method looks like this:</p>
  
  <pre><code>    load():void {
          this.http.get(SVG_LIST)
              .map(res =&gt; {
                  return &lt;Array&lt;string&gt;&gt; res.json().files;
              })
              .catch(this.handleError);
      }
  </code></pre>
  
  <p>What do I chain onto the <code>map</code> method?</p>