title: fromPromise does not exist on type Observable
body: <p>In Angular 2 using rxjs I was trying to convert a Promise to Observable. As many of online guides showed I used <code>fromPromise</code> on <code>Observable</code>. Which throws error:</p>
  
  <pre><code>Property 'fromPromise' does not exist on type 'typeof Observable'.
  </code></pre>
  
  <p>Observable was imported like:</p>
  
  <pre><code>import { Observable } from "rxjs/Observable";
  </code></pre>
  
  <p>trying to import <code>fromPromise</code> like other operators results in error:</p>
  
  <pre><code>import 'rxjs/add/operator/fromPromise';
  </code></pre>
  
  <p>even if I suppress typescript error it still results in error:</p>
  
  <pre><code>(&lt;any&gt;Observable).fromPromise
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>Uncaught (in promise): TypeError: __WEBPACK_IMPORTED_MODULE_3_rxjs_Observable__.Observable.fromPromise is not a function
  </code></pre>
  
  <p>Somewhat similar issue was reported on rxjs repo <a href="https://github.com/ReactiveX/rxjs/issues/1507" rel="noreferrer">here</a> but there is no solution there either.</p>