Update to Drupal 8.1.1. For more information, see https://www.drupal.org/node/2718713

core/lib/Drupal/Core/Extension/InfoParserInterface.php

@@ -60,4 +60,3 @@ interface InfoParserInterface  {
   public function parse($filename);
 
 }
-

core/lib/Drupal/Core/Extension/ModuleInstallerInterface.php

@@ -82,4 +82,3 @@ interface ModuleInstallerInterface {
   public function validateUninstall(array $module_list);
 
 }
-

core/lib/Drupal/Core/Extension/module.api.php

@@ -894,6 +894,9 @@ function hook_updater_info_alter(&$updaters) {
  * Module dependencies do not belong to these installation requirements,
  * but should be defined in the module's .info.yml file.
  *
+ * During installation (when $phase == 'install'), if you need to load a class
+ * from your module, you'll need to include the class file directly.
+ *
  * The 'runtime' phase is not limited to pure installation requirements
  * but can also be used for more general status information like maintenance
  * tasks and security issues.