11 files declare their use of Yaml in Drupal 8

DistributionProfileExistingSettingsTest.php in core/tests/Drupal/FunctionalTests/Installer/DistributionProfileExistingSettingsTest.php

drupal-8.file_formatters_update_2677990.php in core/modules/file/tests/fixtures/update/drupal-8.file_formatters_update_2677990.php

Contains database additions to drupal-8.bare.standard.php.gz for testing the upgrade path of https://www.drupal.org/node/2677990.

drupal-8.update-test-block-disabled-2513534.php in core/modules/system/tests/fixtures/update/drupal-8.update-test-block-disabled-2513534.php

Contains database additions to drupal-8.bare.standard.php.gz for testing the upgrade path of https://www.drupal.org/node/2513534.

ExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php

InstallerExistingConfigTestBase.php in core/tests/Drupal/FunctionalTests/Installer/InstallerExistingConfigTestBase.php

MultipleDistributionsProfileTest.php in core/tests/Drupal/FunctionalTests/Installer/MultipleDistributionsProfileTest.php

UpdateScriptTest.php in core/modules/system/tests/src/Functional/UpdateSystem/UpdateScriptTest.php

Yaml.php in core/lib/Drupal/Core/Serialization/Yaml.php

YamlDirectoryDiscovery.php in core/lib/Drupal/Component/Discovery/YamlDirectoryDiscovery.php

YamlDiscovery.php in core/lib/Drupal/Component/Discovery/YamlDiscovery.php

YamlTest.php in core/tests/Drupal/Tests/Component/Serialization/YamlTest.php