10 files declare their use of NullBackend

CachedStorageTest.php in core/tests/Drupal/Tests/Core/Config/CachedStorageTest.php
ContextDefinitionIsSatisfiedTest.php in core/tests/Drupal/Tests/Core/Plugin/Context/ContextDefinitionIsSatisfiedTest.php
ContextHandlerTest.php in core/tests/Drupal/Tests/Core/Plugin/ContextHandlerTest.php
Contains \Drupal\Tests\Core\Plugin\ContextHandlerTest.
EntityContextDefinitionIsSatisfiedTest.php in core/tests/Drupal/Tests/Core/Plugin/Context/EntityContextDefinitionIsSatisfiedTest.php
NullBackendTest.php in core/tests/Drupal/Tests/Core/Cache/NullBackendTest.php
ProviderRepositoryTest.php in core/modules/media/tests/src/Kernel/ProviderRepositoryTest.php
RecursiveContextualValidatorTest.php in core/tests/Drupal/Tests/Core/TypedData/RecursiveContextualValidatorTest.php
ResourceFetcherTest.php in core/modules/media/tests/src/Unit/ResourceFetcherTest.php
ThemeExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
UpdateBackend.php in core/lib/Drupal/Core/Update/UpdateBackend.php

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