title: AnimatedSwitcher with IndexedStack
body: <p>I have to use IndexedStack to maintain the state of my widgets for my BottomNavigationBar.
  Now i want to use AnimatedSwitcher (or an alternative) to create an animation when i switch tabs.
  I'm having issues getting AnimatedSwitcher to trigger on change of IndexedStack. I'm having IndexedStack as the child of AnimatedSwitcher, which obviously causes AnimatedSwitcher to not trigger because the IndexedStack widget doesn't change, only it's child.</p>
  
  <pre><code>body: AnimatedSwitcher(  
    duration: Duration(milliseconds: 200),  
    child: IndexedStack(  
      children: _tabs.map((t) =&gt; t.widget).toList(),  
      index: _currentIndex,  
    ),  
  )
  </code></pre>
  
  <p>Is there any way around this issue? By manually triggering the AnimatedSwitcher, or by using a different method to create an animation?
  I also tried changing the key, but that obviously resulted in it creating a new IndexedStack everytime the a new state was created, and therefor the states of the tabs was lost as well.</p>