8 files declare their use of NotificationHandler

NotificationHandlerTest.php in core/modules/user/tests/src/Kernel/NotificationHandlerTest.php
user.module in core/modules/user/user.module
User.php in core/modules/user/src/Entity/User.php
UserAuthenticationController.php in core/modules/user/src/Controller/UserAuthenticationController.php
UserCancelForm.php in core/modules/user/src/Form/UserCancelForm.php
UserPasswordForm.php in core/modules/user/src/Form/UserPasswordForm.php
UserRegistrationResource.php in core/modules/user/src/Plugin/rest/resource/UserRegistrationResource.php
UserRegistrationResourceTest.php in core/modules/user/tests/src/Unit/UserRegistrationResourceTest.php

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