15 files declare their use of Yaml

DependencyTest.php in core/modules/system/tests/src/Functional/Module/DependencyTest.php
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
InstallerExistingConfigSyncDirectoryMultilingualTest.php in core/tests/Drupal/FunctionalTests/Installer/InstallerExistingConfigSyncDirectoryMultilingualTest.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
StarterKit.php in core/themes/starterkit_theme/src/StarterKit.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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