diff --git a/core/modules/menu_link_content/src/Entity/MenuLinkContent.php b/core/modules/menu_link_content/src/Entity/MenuLinkContent.php
index d83f871..dc14109 100644
--- a/core/modules/menu_link_content/src/Entity/MenuLinkContent.php
+++ b/core/modules/menu_link_content/src/Entity/MenuLinkContent.php
@@ -200,15 +200,22 @@ public function postSave(EntityStorageInterface $storage, $update = TRUE) {
 
     // The menu link can just be updated if there is already an menu link entry
     // on both entity and menu link plugin level.
-    if ($update && $menu_link_manager->getDefinition($this->getPluginId())) {
+    $definition = $this->getPluginDefinition();
+    // Even when $update is FALSE, for top level links it is possible the link
+    // already is in the storage because of the getPluginDefinition() call
+    // above, see https://www.drupal.org/node/2605684#comment-10515450 for the
+    // call chain. Because of this the $update flag is ignored and only the
+    // existence of the definition (equals to being in the tree storage) is
+    // checked.
+    if ($menu_link_manager->getDefinition($this->getPluginId(), FALSE)) {
       // When the entity is saved via a plugin instance, we should not call
       // the menu tree manager to update the definition a second time.
       if (!$this->insidePlugin) {
-        $menu_link_manager->updateDefinition($this->getPluginId(), $this->getPluginDefinition(), FALSE);
+        $menu_link_manager->updateDefinition($this->getPluginId(), $definition, FALSE);
       }
     }
     else {
-      $menu_link_manager->addDefinition($this->getPluginId(), $this->getPluginDefinition());
+      $menu_link_manager->addDefinition($this->getPluginId(), $definition);
     }
   }
 
diff --git a/core/modules/simpletest/src/RouteProvider.php b/core/modules/simpletest/src/RouteProvider.php
new file mode 100644
index 0000000..acff99a
--- /dev/null
+++ b/core/modules/simpletest/src/RouteProvider.php
@@ -0,0 +1,114 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\ProxyClass\Routing\RouteProvider.
+ */
+
+namespace Drupal\simpletest;
+
+use Drupal\Core\Routing\PreloadableRouteProviderInterface;
+use Symfony\Cmf\Component\Routing\PagedRouteProviderInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Rebuilds the router when the provider is instantiated.
+ */
+class RouteProvider implements PreloadableRouteProviderInterface, PagedRouteProviderInterface {
+
+  use \Drupal\Core\DependencyInjection\DependencySerializationTrait;
+
+  /**
+   * Loads the real route provider from the container and rebuilds the router.
+   *
+   * @return \Drupal\Core\Routing\PreloadableRouteProviderInterface|\Symfony\Cmf\Component\Routing\PagedRouteProviderInterface|\Symfony\Component\EventDispatcher\EventSubscriberInterface
+   *   The route provider.
+   */
+  protected function lazyLoadItself() {
+    if (!isset($this->service)) {
+      $container = \Drupal::getContainer();
+      $this->service = $container->get('simpletest.router.route_provider');
+      $container->get('router.builder')->rebuild();
+    }
+
+    return $this->service;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRouteCollectionForRequest(Request $request) {
+    return $this->lazyLoadItself()->getRouteCollectionForRequest($request);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRouteByName($name) {
+    return $this->lazyLoadItself()->getRouteByName($name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function preLoadRoutes($names){
+    return $this->lazyLoadItself()->preLoadRoutes($names);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRoutesByNames($names) {
+    return $this->lazyLoadItself()->getRoutesByNames($names);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCandidateOutlines(array $parts) {
+    return $this->lazyLoadItself()->getCandidateOutlines($parts);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRoutesByPattern($pattern) {
+    return $this->lazyLoadItself()->getRoutesByPattern($pattern);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function routeProviderRouteCompare(array $a, array $b) {
+    return $this->lazyLoadItself()->routeProviderRouteCompare($a, $b);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getAllRoutes() {
+    return $this->lazyLoadItself()->getAllRoutes();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    return $this->lazyLoadItself()->reset();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRoutesPaged($offset, $length = NULL) {
+    return $this->lazyLoadItself()->getRoutesPaged($offset, $length);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRoutesCount() {
+    return $this->lazyLoadItself()->getRoutesCount();
+  }
+
+}
diff --git a/core/modules/simpletest/src/TestServiceProvider.php b/core/modules/simpletest/src/TestServiceProvider.php
index 64dedb8..5a1a398 100644
--- a/core/modules/simpletest/src/TestServiceProvider.php
+++ b/core/modules/simpletest/src/TestServiceProvider.php
@@ -8,9 +8,11 @@
 namespace Drupal\simpletest;
 
 use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\DependencyInjection\ServiceModifierInterface;
 use Drupal\Core\DependencyInjection\ServiceProviderInterface;
+use Symfony\Component\DependencyInjection\Definition;
 
-class TestServiceProvider implements ServiceProviderInterface {
+class TestServiceProvider implements ServiceProviderInterface, ServiceModifierInterface {
 
   /**
    * @var \Drupal\simpletest\TestBase;
@@ -25,4 +27,21 @@ function register(ContainerBuilder $container) {
       static::$currentTest->containerBuild($container);
     }
   }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function alter(ContainerBuilder $container) {
+    if (static::$currentTest instanceof KernelTestBase) {
+      // While $container->get() does a recursive resolve, getDefinition() does
+      // not, so do it ourselves.
+      foreach (['router.route_provider' => 'RouteProvider'] as $original_id => $class) {
+        for ($id = $original_id; $container->hasAlias($id); $id = (string) $container->getAlias($id));
+        $definition = $container->getDefinition($id);
+        $definition->clearTag('needs_destruction');
+        $container->setDefinition("simpletest.$original_id", $definition);
+        $container->setDefinition($id, new Definition('Drupal\simpletest\\' . $class));
+      }
+    }
+  }
 }