12 files declare their use of Yaml in Drupal 9

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

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

FrontMatterTest.php in core/tests/Drupal/Tests/Component/FrontMatter/FrontMatterTest.php

GenerateTheme.php in core/lib/Drupal/Core/Command/GenerateTheme.php

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

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