12 files declare their use of HandlerBase in Zircon Profile 8

AreaPluginBase.php in core/modules/views/src/Plugin/views/area/AreaPluginBase.php

Contains \Drupal\views\Plugin\views\area\AreaPluginBase.

ArgumentPluginBase.php in core/modules/views/src/Plugin/views/argument/ArgumentPluginBase.php

Contains \Drupal\views\Plugin\views\argument\ArgumentPluginBase.

FieldPluginBase.php in core/modules/views/src/Plugin/views/field/FieldPluginBase.php

Contains \Drupal\views\Plugin\views\field\FieldPluginBase.

FilterPluginBase.php in core/modules/views/src/Plugin/views/filter/FilterPluginBase.php

Contains \Drupal\views\Plugin\views\filter\FilterPluginBase.

HandlerAllTest.php in core/modules/views/src/Tests/Handler/HandlerAllTest.php

Contains \Drupal\views\Tests\Handler\HandlerAllTest.

HandlerBaseTest.php in core/modules/views/tests/src/Unit/Plugin/HandlerBaseTest.php

Contains \Drupal\Tests\views\Unit\Plugin\HandlerBaseTest.

HandlerTest.php in core/modules/views/src/Tests/Handler/HandlerTest.php

Contains \Drupal\views\Tests\Handler\HandlerTest.

ManyToOneHelper.php in core/modules/views/src/ManyToOneHelper.php

Contains \Drupal\views\ManyToOneHelper.

RelationshipPluginBase.php in core/modules/views/src/Plugin/views/relationship/RelationshipPluginBase.php

Contains \Drupal\views\Plugin\views\relationship\RelationshipPluginBase.

SortPluginBase.php in core/modules/views/src/Plugin/views/sort/SortPluginBase.php

Contains \Drupal\views\Plugin\views\sort\SortPluginBase.

Sql.php in core/modules/views/src/Plugin/views/query/Sql.php

Contains \Drupal\views\Plugin\views\query\Sql.

ViewsHandlerManager.php in core/modules/views/src/Plugin/ViewsHandlerManager.php

Contains \Drupal\views\Plugin\ViewsHandlerManager.