21 files declare their use of Yaml

ConfigConfiguratorTest.php in core/tests/Drupal/KernelTests/Core/Recipe/ConfigConfiguratorTest.php
ConfigValidationTest.php in core/tests/Drupal/KernelTests/Core/Recipe/ConfigValidationTest.php
ContentImportTest.php in core/tests/Drupal/FunctionalTests/DefaultContent/ContentImportTest.php
DependencyTest.php in core/modules/system/tests/src/Functional/Module/DependencyTest.php
DistributionProfileExistingSettingsTest.php in core/tests/Drupal/FunctionalTests/Installer/DistributionProfileExistingSettingsTest.php
entity-id-argument.php in core/modules/views/tests/fixtures/update/entity-id-argument.php
Test fixture.
ExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
Finder.php in core/lib/Drupal/Core/DefaultContent/Finder.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
Recipe.php in core/lib/Drupal/Core/Recipe/Recipe.php
RecipeTestTrait.php in core/tests/Drupal/FunctionalTests/Core/Recipe/RecipeTestTrait.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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