6 files declare their use of RouteProviderInterface

MenuLinkStorageController.php in drupal/core/modules/menu_link/lib/Drupal/menu_link/MenuLinkStorageController.php
Contains \Drupal\menu_link\MenuLinkStorageController.
MockRouteProvider.php in drupal/core/modules/system/lib/Drupal/system/Tests/Routing/MockRouteProvider.php
Contains Drupal\system\Tests\Routing\MockRouteProvider.
NestedMatcher.php in drupal/core/vendor/symfony-cmf/routing/Symfony/Cmf/Component/Routing/NestedMatcher/NestedMatcher.php
ProviderBasedGenerator.php in drupal/core/vendor/symfony-cmf/routing/Symfony/Cmf/Component/Routing/ProviderBasedGenerator.php
RouteProvider.php in drupal/core/lib/Drupal/Core/Routing/RouteProvider.php
Contains Drupal\Core\Routing\RouteProvider.
UrlGenerator.php in drupal/core/lib/Drupal/Core/Routing/UrlGenerator.php
Contains Drupal\Core\Routing\UrlGenerator.