7 files declare their use of RouterInterface in Zircon Profile 8

AccessAwareRouterInterface.php in core/lib/Drupal/Core/Routing/AccessAwareRouterInterface.php

Contains \Drupal\Core\Routing\AccessAwareRouterInterface.

ChainedRouterInterface.php in vendor/symfony-cmf/routing/ChainedRouterInterface.php

ChainRouter.php in vendor/symfony-cmf/routing/ChainRouter.php

ChainRouterInterface.php in vendor/symfony-cmf/routing/ChainRouterInterface.php

ChainRouterTest.php in vendor/symfony-cmf/routing/Tests/Routing/ChainRouterTest.php

DynamicRouter.php in vendor/symfony-cmf/routing/DynamicRouter.php

TestRouterInterface.php in core/tests/Drupal/Tests/Core/Routing/TestRouterInterface.php

Contains \Drupal\Tests\Core\Routing\TestRouterInterface.