title: Angular 6 upgrade: debounceTime is not property of Subject
body: <p>I am attempting to upgrade my app from Angular 5 to Angular 6. I followed the steps on the <a href="https://update.angular.io/" rel="noreferrer">https://update.angular.io/</a>
  At least i think i did. </p>
  
  <p>The Error is :</p>
  
  <pre><code>Property 'debounceTime' does not exist on type 'Subject&lt;string&gt;'.
  </code></pre>
  
  <p>Also my components lost the debounceTime import. I think the ng update removed it. </p>