title: Impossible to import mx.controls.Alert
body: <p>I got a strange problem.
  Just using Flash Builder 4.5 and trying to use the Alert Popup for some user messages.
  But when i use it, it told me : </p>
  
  <p>"Importation not found" (with a ? logo)</p>
  
  <p>Here is the code : </p>
  
  <pre><code>&lt;fx:Script&gt;
      &lt;![CDATA[
          import mx.controls.Alert;
          import mx.events.CloseEvent;
          import mx.managers.PopUpManager;
  </code></pre>