diff --git a/core/lib/Drupal/Core/Routing/RouteProvider.php b/core/lib/Drupal/Core/Routing/RouteProvider.php
index ccfb070..77b9449 100644
--- a/core/lib/Drupal/Core/Routing/RouteProvider.php
+++ b/core/lib/Drupal/Core/Routing/RouteProvider.php
@@ -319,11 +319,12 @@ public function getRoutesByPattern($pattern) {
   }
 
   /**
-   * Get all routes which match a certain pattern.
+   * Get all routes which match a certain path pattern.
+   *
+   * The path will be converted to lowercase before performing the match.
    *
    * @param string $path
-   *   The route pattern to search for (contains % as placeholders) and it will
-   *   be converted to lowercase.
+   *   The route path pattern to search for (contains % as placeholders).
    *
    * @return \Symfony\Component\Routing\RouteCollection
    *   Returns a route collection of matching routes.
diff --git a/core/modules/system/src/Tests/Routing/RouterTest.php b/core/modules/system/src/Tests/Routing/RouterTest.php
index 277f383..0b24960 100644
--- a/core/modules/system/src/Tests/Routing/RouterTest.php
+++ b/core/modules/system/src/Tests/Routing/RouterTest.php
@@ -213,9 +213,12 @@ public function testRouterMatching() {
 
   /**
    * Tests the case insensitivity of route paths and path variable handling.
+   *
+   * The un-routed URLs are constructed using base: so that we are sure they are
+   * used as written without being processed by the routing system.
    */
   public function testRoutePathMixedCase() {
-    $this->drupalGet('router_test/TEST14/1');
+    $this->drupalGet(Url::fromUri('base:router_test/TEST14/1'));
     $this->assertResponse(200);
     $this->assertText('User route "entity.user.canonical" was matched.');
 
@@ -225,15 +228,15 @@ public function testRoutePathMixedCase() {
     // Verify that the route path is stored as lowercase despite being defined
     // with mixed case in the YAML file.
     $this->assertIdentical('/router_test/mixedcase2', $route2->getPath());
-    $this->drupalGet('/router_test/mixEDCASE2');
+    $this->drupalGet(Url::fromUri('base:router_test/mixEDCASE2'));
     $this->assertResponse(200);
     $this->assertRaw('test2', 'The correct string was returned because the route was successful.');
 
-    $this->drupalGet('/router_TEST/MIXEDcase2');
+    $this->drupalGet(Url::fromUri('base:router_TEST/MIXEDcase2'));
     $this->assertResponse(200);
     $this->assertRaw('test2', 'The correct string was returned because the route was successful.');
 
-    $this->drupalGet('/router_test/mixedcase2');
+    $this->drupalGet(Url::fromUri('base:router_test/mixedcase2'));
     $this->assertResponse(200);
     $this->assertRaw('test2', 'The correct string was returned because the route was successful.');
 
@@ -244,17 +247,33 @@ public function testRoutePathMixedCase() {
 
     // Verify that data in variable path parts is retained as the original,
     // mixed-case value. Matches route path /router_test/mixedcase3/{value}
-    $this->drupalGet('router_test/mixedcase3/mixedCASEstring');
+    $this->drupalGet(Url::fromUri('base:router_test/mixedcase3/mixedCASEstring'));
     $this->assertResponse(200);
     $this->assertRaw('mixedCASEstring', 'The correct string was returned because the route was successful.');
 
-    $this->drupalGet('router_test/mixedcase3/mixedcasestring');
+    $this->drupalGet(Url::fromUri('base:router_test/mixedcase3/mixedcasestring'));
     $this->assertResponse(200);
     $this->assertRaw('mixedcasestring', 'The correct string was returned because the route was successful.');
 
-    $this->drupalGet('router_test/mixedcase3/MIXEDCASESTRING');
+    $this->drupalGet(Url::fromUri('base:router_test/mixedcase3/MIXEDCASESTRING'));
     $this->assertResponse(200);
     $this->assertRaw('MIXEDCASESTRING', 'The correct string was returned because the route was successful.');
+    // Test routes added by \Drupal\router_test\RouteTestSubscriber.
+    // Check that a dynamically added route has path changed to lower-case.
+    $route4 = $route_provider->getRouteByName('router_test.mixedcase.4');
+    $this->assertIdentical('/router_test/mixedcase4', $route4->getPath());
+    $this->drupalGet(Url::fromUri('base:router_TEST/MixedCASE4'));
+    $this->assertResponse(200);
+    // Check that a route added dynamically and altered has path lower-cased.
+    $route5 = $route_provider->getRouteByName('router_test.mixedcase.5');
+    $this->assertIdentical('/router_test/mixedcase5/altered', $route5->getPath());
+    $this->drupalGet(Url::fromUri('base:router_test/MixedCASE5/altereD'));
+    $this->assertResponse(200);
+    // Check that a route added during the alter event has path lower-cased.
+    $route6 = $route_provider->getRouteByName('router_test.mixedcase.6');
+    $this->assertIdentical('/router_test/mixedcase6', $route6->getPath());
+    $this->drupalGet(Url::fromUri('base:router_test/MixedCASE6'));
+    $this->assertResponse(200);
   }
 
   /**
diff --git a/core/modules/system/tests/modules/router_test_directory/src/RouteTestSubscriber.php b/core/modules/system/tests/modules/router_test_directory/src/RouteTestSubscriber.php
index 3bc703b..72b824b 100644
--- a/core/modules/system/tests/modules/router_test_directory/src/RouteTestSubscriber.php
+++ b/core/modules/system/tests/modules/router_test_directory/src/RouteTestSubscriber.php
@@ -7,21 +7,82 @@
 
 namespace Drupal\router_test;
 
-use Drupal\Core\Routing\RouteSubscriberBase;
-use Symfony\Component\Routing\RouteCollection;
+use Drupal\Core\Routing\RouteBuildEvent;
+use Drupal\Core\Routing\RoutingEvents;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\Routing\Route;
 
 /**
  * Listens to the dynamic route event and add a test route.
  */
-class RouteTestSubscriber extends RouteSubscriberBase {
+class RouteTestSubscriber implements EventSubscriberInterface {
 
   /**
    * {@inheritdoc}
    */
-  protected function alterRoutes(RouteCollection $collection) {
+  public static function getSubscribedEvents() {
+    $events[RoutingEvents::DYNAMIC] = 'onDynamicRoutes';
+    $events[RoutingEvents::ALTER] = 'onAlterRoutes';
+    return $events;
+  }
+
+  /**
+   * Adds dynamic routes for a specific collection from an event.
+   *
+   * @param \Drupal\Core\Routing\RouteBuildEvent $event
+   *   The route build event.
+   */
+  public function onDynamicRoutes(RouteBuildEvent $event) {
+    $collection = $event->getRouteCollection();
+    $route = new Route(
+      '/router_test/mixedCASE4',
+      [
+        '_controller' => '\Drupal\router_test\TestControllers::test2',
+      ],
+      [
+        '_access' => 'TRUE',
+      ]
+    );
+    $collection->add("router_test.mixedcase.4", $route);
+    $route = new Route(
+      '/router_test/tobealtered',
+      [
+        '_controller' => '\Drupal\router_test\TestControllers::test2',
+      ],
+      [
+        '_access' => 'TRUE',
+      ]
+    );
+    $collection->add("router_test.mixedcase.5", $route);
+  }
+
+  /**
+   * Alters existing routes for a specific collection from an event.
+   *
+   * @param \Drupal\Core\Routing\RouteBuildEvent $event
+   *   The route build event.
+   */
+  public function onAlterRoutes(RouteBuildEvent $event) {
+    $collection = $event->getRouteCollection();
     $route = $collection->get('router_test.6');
     // Change controller method from test1 to test5.
     $route->setDefault('_controller', '\Drupal\router_test\TestControllers::test5');
+    // Change a route path from lower case to mixed case.
+    $route = $collection->get('router_test.mixedcase.5');
+    $route->setPath('/router_tesT/MIXedcase5/ALTERED');
+    // We can also add routes in the alter, which happens in classes like
+    // \Drupal\content_translation\Routing\ContentTranslationRouteSubscriber
+    // that need to add routes based on other routes that are added dynamically.
+    $route = new Route(
+      '/router_test/mixedCASE6',
+      [
+        '_controller' => '\Drupal\router_test\TestControllers::test2',
+      ],
+      [
+        '_access' => 'TRUE',
+      ]
+    );
+    $collection->add("router_test.mixedcase.6", $route);
   }
 
 }