9 files declare their use of StatementInterface in Drupal 8

DatabaseLegacyTest.php in core/tests/Drupal/KernelTests/Core/Database/DatabaseLegacyTest.php

EmptyStatementTest.php in core/tests/Drupal/Tests/Core/Database/EmptyStatementTest.php

FetchTest.php in core/tests/Drupal/KernelTests/Core/Database/FetchTest.php

HelpSearch.php in core/modules/help_topics/src/Plugin/Search/HelpSearch.php

node.module in core/modules/node/node.module

The core module that allows content to be submitted to the site.

NodeSearch.php in core/modules/node/src/Plugin/Search/NodeSearch.php

PagerPluginBaseTest.php in core/modules/views/tests/src/Unit/Plugin/pager/PagerPluginBaseTest.php

Contains \Drupal\Tests\views\Unit\Plugin\pager\PagerPluginBaseTest.

SqlBaseTest.php in core/modules/migrate/tests/src/Kernel/SqlBaseTest.php

Contains \Drupal\Tests\migrate\Kernel\SqlBaseTest.

Statement.php in core/lib/Drupal/Core/Database/Driver/sqlite/Statement.php