title: rxjs5 toPromise not working
body: <p>In the following example <code>toPromise</code> does not work:</p>
  
  <p><a href="https://jsfiddle.net/tossp/nmf9jg32/" rel="noreferrer">https://jsfiddle.net/tossp/nmf9jg32/</a></p>
  
  <p>My code:</p>
  
  <pre><code>function getPostData() {
      return fetch('https://jsonplaceholder.typicode.com/posts/1')
      .then(res =&gt; res.json())
  }
  var source = Rx.Observable.fromEvent(document.body, 'click');
  
  var example = source.concatMap(
              e =&gt; Rx.Observable.from(getPostData()), 
              (e, res, eIndex, resIndex) =&gt; res.title);
  
  example.subscribe({
      next: (value) =&gt; { console.log('subscribe!!!',value); },
      error: (err) =&gt; { console.log('Error: ' + err); },
      complete: () =&gt; { console.log('complete'); }
  });
  example.do((value)=&gt;console.log('do!!!',value)).toPromise().then((value)=&gt;console.log('toPromise!!!',value));
  </code></pre>