10 files declare their use of HandlerBase in Views (for Drupal 7) 8.3

AreaPluginBase.php in lib/Drupal/views/Plugin/views/area/AreaPluginBase.php

Definition of Drupal\views\Plugin\views\area\AreaPluginBase.

ArgumentPluginBase.php in lib/Drupal/views/Plugin/views/argument/ArgumentPluginBase.php

Definition of Drupal\views\Plugin\views\argument\ArgumentPluginBase.

FieldPluginBase.php in lib/Drupal/views/Plugin/views/field/FieldPluginBase.php

Definition of Drupal\views\Plugin\views\field\FieldPluginBase.

FilterPluginBase.php in lib/Drupal/views/Plugin/views/filter/FilterPluginBase.php

Definition of Drupal\views\Plugin\views\filter\FilterPluginBase.

HandlerAllTest.php in lib/Drupal/views/Tests/Handler/HandlerAllTest.php

Definition of Drupal\views\Tests\Handler\HandlerAllTest.

HandlerTest.php in lib/Drupal/views/Tests/Handler/HandlerTest.php

Definition of Drupal\views\Tests\Handler\HandlerTest.

ManyToOneHelper.php in lib/Drupal/views/ManyToOneHelper.php

Definition of Drupal\views\ManyToOneHelper.

RelationshipPluginBase.php in lib/Drupal/views/Plugin/views/relationship/RelationshipPluginBase.php

Definition of Drupal\views\Plugin\views\relationship\RelationshipPluginBase.

SortPluginBase.php in lib/Drupal/views/Plugin/views/sort/SortPluginBase.php

Definition of Drupal\views\Plugin\views\sort\SortPluginBase.

Sql.php in lib/Drupal/views/Plugin/views/query/Sql.php

Definition of Drupal\views\Plugin\views\query\Sql.