8 files declare their use of FallbackPluginManagerInterface

BlockManager.php in core/lib/Drupal/Core/Block/BlockManager.php
FieldTypeCategoryManager.php in core/lib/Drupal/Core/Field/FieldTypeCategoryManager.php
FilterPluginManager.php in core/modules/filter/src/FilterPluginManager.php
PluginExistsConstraintValidator.php in core/lib/Drupal/Core/Plugin/Plugin/Validation/Constraint/PluginExistsConstraintValidator.php
PluginExistsConstraintValidatorTest.php in core/tests/Drupal/KernelTests/Core/Plugin/PluginExistsConstraintValidatorTest.php
SelectionPluginManager.php in core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginManager.php
StubFallbackPluginManager.php in core/tests/Drupal/Tests/Component/Plugin/StubFallbackPluginManager.php
ViewsHandlerManager.php in core/modules/views/src/Plugin/ViewsHandlerManager.php

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