title: Angular 5 to 6 Upgrade: Property 'map' does not exist on type Observable
body: <p>Ive upgraded my angular application from version 5 to 6 and im getting this error from the following code.</p>
  
  <pre><code>  const request = this.evidenceService.get().map((res) =&gt; res.data)
                  .catch(error =&gt; Observable.of(null));
  </code></pre>
  
  <blockquote>
    <p>Property 'map' does not exist on type 'Observable'.</p>
  </blockquote>