6 files declare their use of YamlDiscovery in Drupal 8

MigrationState.php in core/modules/migrate_drupal/src/MigrationState.php

PermissionHandler.php in core/modules/user/src/PermissionHandler.php

RouteBuilder.php in core/lib/Drupal/Core/Routing/RouteBuilder.php

RouteBuilderTest.php in core/tests/Drupal/Tests/Core/Routing/RouteBuilderTest.php

Contains \Drupal\Tests\Core\Routing\RouteBuilderTest.

YamlDiscovery.php in core/lib/Drupal/Core/Plugin/Discovery/YamlDiscovery.php

YamlDiscoveryTest.php in core/tests/Drupal/Tests/Core/Discovery/YamlDiscoveryTest.php