9 files declare their use of PluginBase

JoinPluginBase.php in drupal/core/modules/views/lib/Drupal/views/Plugin/views/join/JoinPluginBase.php
Definition of Drupal\views\Plugin\views\join\JoinPluginBase.
MockLayoutBlock.php in drupal/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/Plugin/plugin_test/mock_block/MockLayoutBlock.php
Definition of Drupal\plugin_test\Plugin\plugin_test\mock_block\MockLayoutBlock.
MockMenuBlock.php in drupal/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/Plugin/plugin_test/mock_block/MockMenuBlock.php
Definition of Drupal\plugin_test\Plugin\plugin_test\mock_block\MockMenuBlock.
MockTestBlock.php in drupal/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/Plugin/plugin_test/mock_block/MockTestBlock.php
Contains Drupal\plugin_test\Plugin\plugin_test\mock_block\MockTestBlock.
MockUserLoginBlock.php in drupal/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/Plugin/plugin_test/mock_block/MockUserLoginBlock.php
Definition of Drupal\plugin_test\Plugin\plugin_test\mock_block\MockUserLoginBlock.
PluginBase.php in drupal/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php
Definition of Drupal\views\Plugin\views\PluginBase.
PluginSettingsBase.php in drupal/core/modules/field/lib/Drupal/field/Plugin/PluginSettingsBase.php
Definition of Drupal\field\Plugin\PluginSettingsBase.
ResourceBase.php in drupal/core/modules/rest/lib/Drupal/rest/Plugin/ResourceBase.php
Definition of Drupal\rest\Plugin\ResourceBase.
StaticLayout.php in drupal/core/modules/layout/lib/Drupal/layout/Plugin/layout/layout/StaticLayout.php
Definition of Drupal\layout\Plugin\layout\layout\StaticLayout.