title: mx.collections missing? ArrayCollection() in Flex4 (Flash Builder 4)
body: <p>I have some code that uses an ArrayCollection, it imports:</p>
  
  <pre><code>import mx.collections.ArrayCollection;
  </code></pre>
  
  <p>then:</p>
  
  <pre><code>static protected var myAC:ArrayCollection = new ArrayCollection();
  </code></pre>
  
  <p>Seems straightforward, but in my project the import (and thus ArrayCollection) are not found/defined!</p>
  
  <p>I am using Flash Builder 4 (Flex 4) - it's an ActionScript project, and project properties is set to Flex SDK 4.0.   Everything should be default settings.</p>
  
  <p>When I type "import mx." and press control-space (autocomplete), I see a list of choices such as core, geom and several others, but no 'collections'.</p>
  
  <p>I must be missing something?</p>