Autoloader expected class Symfony2
Missing (or short) PHP opening tag can cause that error too. Yes it sounds funny, but if you just follow Symfony example and copy/paste whole class you may not notice that (as I didn't).
This is a little old now, but in case anyone else gets here by search:This is undoubtedly a cache issue. Manually delete the contents of app/cache/dev and app/cache/prod, and everything should resolve.
If anyone else has this issue and it doesn't go away by clearing the cache, then I recommend removing the entire folder with your composer dependencies and re-executing composer install.
It worked for me :) .