11 files declare their use of Yaml

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

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