Fixing Class Not Found error
In addition to the usual cache rebuild, the additional need to restart the web server is due to a decision in Drupal 8 to be 'fast by default' use the APC Classloader if it is available.  It is faster, but it doesn't necessarily catch changes as you make them.  To not have to restart your web server during development, you can set your local environment to not try to use a better-for-production class loader.

For instance, in local.settings.php add:

$settings['class_loader_auto_detect'] = FALSE;


Via, and more background information at, what to do when developing a Drupal 8 module and a class file just isn't being loaded even though it is definitely there.