diff -u b/core/lib/Drupal/Core/Routing/MatcherDumper.php b/core/lib/Drupal/Core/Routing/MatcherDumper.php
--- b/core/lib/Drupal/Core/Routing/MatcherDumper.php
+++ b/core/lib/Drupal/Core/Routing/MatcherDumper.php
@@ -67,0 +68,12 @@
+   * {@inheritdoc}
+   */
+  public function addRoutes(RouteCollection $routes) {
+    if (empty($this->routes)) {
+      $this->routes = $routes;
+    }
+    else {
+      $this->routes->addCollection($routes);
+    }
+  }
+
+  /**
diff -u b/core/lib/Drupal/Core/Routing/MatcherDumperInterface.php b/core/lib/Drupal/Core/Routing/MatcherDumperInterface.php
--- b/core/lib/Drupal/Core/Routing/MatcherDumperInterface.php
+++ b/core/lib/Drupal/Core/Routing/MatcherDumperInterface.php
@@ -25,2 +25,10 @@
 
+  /**
+   * Adds additional routes to be dumped.
+   *
+   * @param \Symfony\Component\Routing\RouteCollection $routes
+   *   A collection of routes to add to this dumper.
+   */
+  public function addRoutes(RouteCollection $routes);
+
 }
diff -u b/core/modules/system/lib/Drupal/system/Tests/Routing/MatcherDumperTest.php b/core/modules/system/lib/Drupal/system/Tests/Routing/MatcherDumperTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/Routing/MatcherDumperTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Routing/MatcherDumperTest.php
@@ -86,12 +86,12 @@
     $route = new Route('test');
     $collection = new RouteCollection();
     $collection->add('test_route', $route);
-    $dumper->setRoutes($collection);
+    $dumper->addRoutes($collection);
 
     $route = new Route('test2');
     $collection2 = new RouteCollection();
     $collection2->add('test_route2', $route);
-    $dumper->setRoutes($collection2);
+    $dumper->addRoutes($collection2);
 
     // Merge the two collections together so we can test them.
     $collection->addCollection(clone $collection2);