31 files declare their use of Yaml in Drupal 9

addTourLegacyConfig.php in core/modules/tour/tests/fixtures/addTourLegacyConfig.php

Adds deprecated tour config for testing updates.

ConfigController.php in core/modules/config/src/Controller/ConfigController.php

ConfigExportUITest.php in core/modules/config/tests/src/Functional/ConfigExportUITest.php

ConfigInstallProfileUnmetDependenciesTest.php in core/modules/config/tests/src/Functional/ConfigInstallProfileUnmetDependenciesTest.php

ConfigManager.php in core/lib/Drupal/Core/Config/ConfigManager.php

ConfigSingleExportForm.php in core/modules/config/src/Form/ConfigSingleExportForm.php

ConfigSingleImportExportTest.php in core/modules/config/tests/src/Functional/ConfigSingleImportExportTest.php

ConfigSingleImportForm.php in core/modules/config/src/Form/ConfigSingleImportForm.php

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

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

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

drupal-8.action-3022401.php in core/modules/action/tests/fixtures/update/drupal-8.action-3022401.php

Contains database additions to for testing upgrade path for action settings.

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

EntityTypeWithoutViewsDataTest.php in core/modules/datetime_range/tests/src/Kernel/Views/EntityTypeWithoutViewsDataTest.php

FileStorage.php in core/lib/Drupal/Core/Config/FileStorage.php

FileStorageTest.php in core/tests/Drupal/KernelTests/Core/Config/Storage/FileStorageTest.php

FunctionalTestSetupTrait.php in core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php

HelpTopicDiscovery.php in core/modules/help_topics/src/HelpTopicDiscovery.php

HelpTopicTwigLoader.php in core/modules/help_topics/src/HelpTopicTwigLoader.php

InfoParserDynamic.php in core/lib/Drupal/Core/Extension/InfoParserDynamic.php

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

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

LibraryDiscoveryParser.php in core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php

ModuleInstaller.php in core/lib/Drupal/Core/Extension/ModuleInstaller.php

ModulesListFormWebTest.php in core/modules/system/tests/src/Functional/Form/ModulesListFormWebTest.php

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

ThemeUiTest.php in core/modules/system/tests/src/Functional/Theme/ThemeUiTest.php

TwigEnvironment.php in core/lib/Drupal/Core/Template/TwigEnvironment.php

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

YamlFileLoader.php in core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php

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