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I think, it's worth mentioning that the following clauses should be added at the beginning of TutorialBlock.php :

use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\Component\Plugin\PluginManagerInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

I don't see it in the previous lesson about blocks (http://www.jpstacey.info/blog/2016-09-16/drupal-8-api-blocks) but it is neccessary to make the TutorialBlock works properly.