As part of the PHPUnit Initiative the components in the system module that support Simpletest and WebTestBase have been deprecated in favour of components that support BrowserTestBase.
\Drupal\system\Tests\Update\DbUpdatesTrait has been deprecated in favour of \Drupal\FunctionalTests\Update\DbUpdatesTrait. Tests based on BrowserTestBase should use \Drupal\FunctionalTests\Update\DbUpdatesTrait.
\Drupal\system\Tests\Update\UpdatePathTestBase has been deprecated in favour of \Drupal\FunctionalTests\Update\UpdatePathTestBase. New update tests should use the new base class and existing upgrade tests should be converted to use the new base class.
\Drupal\system\Tests\Update\DbUpdatesTrait::includeUpdates has no replacement, the code of that method should be copied to the relevant place when upgrading Simpletests to PHPUnit that need this.
