title: I can no longer use forkjoin with rxjs 5.5.2
body: <p>Recently, I decided to migrate a big project that has been created from scratch with the "Quick Start" of Angular to a version using Angular CLI 1.5.5. Right now, I'm fixing the different issues that arise and I can't get to fix this one.</p>
  
  <p>I read that it was better to use lettable operators of rxjs, which I did and it works very well. However, I also have these lines of code:</p>
  
  <pre class="lang-js prettyprint-override"><code>import { Observable } from "rxjs/Observable";
  import "rxjs/add/observable/forkjoin";
  
  ...
  
  let piecesGrouping$: Observable&lt;IGroupedPiece[]&gt;[] = deliveries.map(delivery =&gt; this._pieceService.getGroupedPieces(delivery.pieces));
  
  Observable
    .forkJoin(...piecesGrouping$)
    .subscribe((groups) =&gt; {
      groups.forEach((group, i) =&gt; deliveries[i].groupedPieces = group);
  
      resolve();
    });
  </code></pre>
  
  <p>They were working very well in the previous version that was using rxjs 5.4.3, now with rxjs 5.5.2, they don't anymore and I get the following error:</p>
  
  <blockquote>
    <p>ERROR Error: Uncaught (in promise): TypeError: Cannot read property
    'apply' of undefined TypeError: Cannot read property 'apply' of
    undefined</p>
  </blockquote>
  
  <p>I tried to replace the spread operator by <code>piecesGrouping$[0], piecesGrouping$[1]</code> as the error does not say anything about <code>forkJoin</code> in itself, then I get:</p>
  
  <blockquote>
    <p>ERROR Error: Uncaught (in promise): TypeError:
    <strong>WEBPACK_IMPORTED_MODULE_8_rxjs_Observable</strong>.a.forkJoin is not a function TypeError:
    <strong>WEBPACK_IMPORTED_MODULE_8_rxjs_Observable</strong>.a.forkJoin is not a function</p>
  </blockquote>
  
  <p>So it looks like that I'm importing <code>forkJoin</code> the wrong way. I tried to import it from "rxjs/observable/forkJoin" but it didn't work either. </p>
  
  <p>What am I missing?</p>