title: In Flutter, how can I change some widget and see it animate to its new size?
body: <p>I would like to change the child of some widget, and then see it animate to the new child's height, also with a fade transition.</p>
  
  <p>I can do that with <code>AnimatedCrossFade</code>, but then I must keep both <code>firstChild</code> and <code>secondChild</code>, which I cannot.</p>
  
  <p>If I use <code>AnimatedSwitcher</code>, it lets me simply change its child, but then it only animates the fade, not the size.</p>
  
  <p>The <code>AnimatedContainer</code> also doesn't work, since I don't know the size of the children in advance.</p>
  
  <p>Is there some widget I am missing that does what I need, and if not, how can I do that without resorting to <code>AnimationController</code>s?</p>