title: Vue2 Transition not working for new items
body: <p>I'm trying to get a <code>transition-group</code> working so that my list items animate when deleted etc... I've got it working so far as when I "splice" an item from the list, it animates nicely. In this example, when I add a list it actually does animate, however you'll see in the console there's an error message:</p>
  
  <p><code>[Vue warn]: &lt;transition-group&gt; children must be keyed: &lt;tr&gt;</code></p>
  
  <p>In my real application, the list item doesn't get rendered, and there isn't an animation. If I remove the <code>transition-group</code> then the list item does successfully get added to the list </p>
  
  <p>Here's the <a href="https://jsfiddle.net/eywraw8t/1481/" rel="nofollow noreferrer">Fiddle</a>. </p>