Consider the following code:

Future<Null> fadeOutAndUpdateState() async {
   await fadeAnimationController.reverse();
   if (!mounted)
     return;
   await sizeAnimationController.reverse();
   if (!mounted)
     return;
   setState(() {
     dismissed = true;
   });
 }

...it would be nice if the "if (!mounted)" calls weren't needed, but right now they are.

One option would be to make controllers complete with an error when they are aborted (or maybe only when disposed?). That would abort the above function. Unfortunately then you'd have to have a function to wrap the call site of the above function to handle (and ignore) the resulting error in the future...