title: Can an AnimatedContainer animate its height?
body: <p>I'd like to animate a gap between two items in a list.  I thought of using an AminatedContainer with a height initially at zero but I'm not familiar with how to make this work.  My code at the moment is:</p>
  
  <pre><code>    new AnimatedContainer(
        duration: const Duration(milliseconds: 200),
        height: App.itemSelected==id ? 50.0 : 0.0,
        curve: Curves.fastOutSlowIn,
      ),
  </code></pre>
  
  <p>That does change the height of the Container but not in an animated way as I had hoped.  Any help would be gratefully received!</p>