7 files declare their use of SelectInterface

Condition.php in drupal/core/modules/field/modules/field_sql_storage/lib/Drupal/field_sql_storage/Entity/Condition.php
Definition of Drupal\field_sql_storage\Query\ConditionSql.
node.admin.inc in drupal/core/modules/node/node.admin.inc
Content administration and module settings user interface.
node.module in drupal/core/modules/node/node.module
The core module that allows content to be submitted to the site.
PagerSelectExtender.php in drupal/core/lib/Drupal/Core/Database/Query/PagerSelectExtender.php
Definition of Drupal\Core\Database\Query\PagerSelectExtender
Tables.php in drupal/core/modules/field/modules/field_sql_storage/lib/Drupal/field_sql_storage/Entity/Tables.php
Definition of Drupal\field_sql_storage\Entity\Tables.
tablesort.inc in drupal/core/includes/tablesort.inc
Functions to aid in the creation of sortable tables.
user.module in drupal/core/modules/user/user.module
Enables the user registration and login system.