diff --git a/core/core.services.yml b/core/core.services.yml
index 796ece3e9b..4aa40bbcbf 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -865,8 +865,8 @@ services:
     class: \Drupal\Core\Routing\Router
     arguments: ['@router.route_provider', '@path.current', '@url_generator']
     tags:
-      - { name: service_collector, tag: non_lazy_route_enhancer, call: addRouteEnhancer }
-      - { name: service_collector, tag: non_lazy_route_filter, call: addRouteFilter }
+      - { name: service_collector, tag: non_lazy_route_enhancer, call: addRouteEnhancerId }
+      - { name: service_collector, tag: non_lazy_route_filter, call: addRouteFilterId }
     calls:
       - [setContext, ['@router.request_context']]
   router.path_roots_subscriber:
diff --git a/core/lib/Drupal/Core/Routing/Router.php b/core/lib/Drupal/Core/Routing/Router.php
index e949c5efb8..0a5b854d77 100644
--- a/core/lib/Drupal/Core/Routing/Router.php
+++ b/core/lib/Drupal/Core/Routing/Router.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Core\Routing;
 
+use Drupal\Core\DependencyInjection\ClassResolverInterface;
 use Drupal\Core\Path\CurrentPathStack;
 use Symfony\Cmf\Component\Routing\Enhancer\RouteEnhancerInterface as BaseRouteEnhancerInterface;
 use Symfony\Cmf\Component\Routing\LazyRouteCollection;
@@ -86,6 +87,13 @@ class Router extends UrlMatcher implements RequestMatcherInterface, RouterInterf
   protected $urlGenerator;
 
   /**
+   * The class resolver.
+   *
+   * @var \Drupal\Core\DependencyInjection\ClassResolverInterface
+   */
+  protected $classResolver;
+
+  /**
    * Constructs a new Router.
    *
    * @param \Symfony\Cmf\Component\Routing\RouteProviderInterface $route_provider
@@ -94,11 +102,14 @@ class Router extends UrlMatcher implements RequestMatcherInterface, RouterInterf
    *   The current path stack.
    * @param \Symfony\Component\Routing\Generator\UrlGeneratorInterface $url_generator
    *   The URL generator.
+   * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
+   *   The class resolver.
    */
-  public function __construct(BaseRouteProviderInterface $route_provider, CurrentPathStack $current_path, BaseUrlGeneratorInterface $url_generator) {
+  public function __construct(BaseRouteProviderInterface $route_provider, CurrentPathStack $current_path, BaseUrlGeneratorInterface $url_generator, ClassResolverInterface $class_resolver) {
     parent::__construct($current_path);
     $this->routeProvider = $route_provider;
     $this->urlGenerator = $url_generator;
+    $this->classResolver = $class_resolver;
   }
 
   /**
@@ -118,6 +129,14 @@ public function addRouteEnhancer(BaseRouteEnhancerInterface $route_enhancer, $pr
   }
 
   /**
+   * Adds a route enhancer service ID.
+   */
+  public function addRouteEnhancerId($id, $priority = 0) {
+    $this->enhancerIds[$priority][] = $priority;
+    return $this;
+  }
+
+  /**
    * Adds a route filter to the list of used route filters.
    *
    * @param \Symfony\Cmf\Component\Routing\NestedMatcher\RouteFilterInterface $route_filter
@@ -135,6 +154,14 @@ public function addRouteFilter(BaseRouteFilterInterface $route_filter, $priority
   }
 
   /**
+   * Adds a route filter service ID.
+   */
+  public function addRouteFilterId($id, $priority = 0) {
+    $this->filterIds[$priority][] = $priority;
+    return $this;
+  }
+
+  /**
    * {@inheritdoc}
    */
   public function match($pathinfo) {
diff --git a/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php b/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
index b635f2ab22..4aea3f1d09 100644
--- a/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
@@ -2,6 +2,8 @@
 
 namespace Drupal\migrate_drupal_ui\Tests;
 
+@trigger_error('\Drupal\migrate_drupal_ui\Tests\MigrateUpgradeTestBase is deprecated in Drupal 8.4.0 and will be removed before Drupal 9.0.0. Use \Drupal\Tests\migrate_drupal_ui\Functional\MigrateUpgradeTestBase instead.', E_USER_DEPRECATED);
+
 use Drupal\Core\Database\Database;
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
 use Drupal\migrate_drupal\MigrationConfigurationTrait;
@@ -9,6 +11,9 @@
 
 /**
  * Provides a base class for testing migration upgrades in the UI.
+ *
+ * @deprecated in Drupal 8.4.0 and will be removed before Drupal 9.0.0. Use
+ *   \Drupal\Tests\migrate_drupal_ui\Functional\MigrateUpgradeTestBase instead.
  */
 abstract class MigrateUpgradeTestBase extends WebTestBase {
   use MigrationConfigurationTrait;
diff --git a/core/modules/migrate_drupal_ui/src/Tests/MigrateAccessTest.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateAccessTest.php
similarity index 82%
rename from core/modules/migrate_drupal_ui/src/Tests/MigrateAccessTest.php
rename to core/modules/migrate_drupal_ui/tests/src/Functional/MigrateAccessTest.php
index 04f2048eac..3c475e2068 100644
--- a/core/modules/migrate_drupal_ui/src/Tests/MigrateAccessTest.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateAccessTest.php
@@ -1,15 +1,15 @@
 <?php
 
-namespace Drupal\migrate_drupal_ui\Tests;
+namespace Drupal\Tests\migrate_drupal_ui\Functional;
 
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests that only user 1 can access the migrate UI.
  *
  * @group migrate_drupal_ui
  */
-class MigrateAccessTest extends WebTestBase {
+class MigrateAccessTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
similarity index 88%
copy from core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
copy to core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
index b635f2ab22..3985327bc1 100644
--- a/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
@@ -1,16 +1,16 @@
 <?php
 
-namespace Drupal\migrate_drupal_ui\Tests;
+namespace Drupal\Tests\migrate_drupal_ui\Functional;
 
 use Drupal\Core\Database\Database;
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
 use Drupal\migrate_drupal\MigrationConfigurationTrait;
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Provides a base class for testing migration upgrades in the UI.
  */
-abstract class MigrateUpgradeTestBase extends WebTestBase {
+abstract class MigrateUpgradeTestBase extends BrowserTestBase {
   use MigrationConfigurationTrait;
 
   /**
@@ -189,6 +189,28 @@ public function testMigrateUpgrade() {
   }
 
   /**
+   * Transforms a nested array into a flat array suitable for BrowserTestBase::drupalPostForm().
+   *
+   * @param array $values
+   *   A multi-dimensional form values array to convert.
+   *
+   * @return array
+   *   The flattened $edit array suitable for BrowserTestBase::drupalPostForm().
+   */
+  protected function translatePostValues(array $values) {
+    $edit = [];
+    // The easiest and most straightforward way to translate values suitable for
+    // BrowserTestBase::drupalPostForm() is to actually build the POST data string
+    // and convert the resulting key/value pairs back into a flat array.
+    $query = http_build_query($values);
+    foreach (explode('&', $query) as $item) {
+      list($key, $value) = explode('=', $item);
+      $edit[urldecode($key)] = urldecode($value);
+    }
+    return $edit;
+  }
+
+  /**
    * Gets the source base path for the concrete test.
    *
    * @return string
diff --git a/core/modules/migrate_drupal_ui/src/Tests/d6/MigrateUpgrade6Test.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
similarity index 92%
rename from core/modules/migrate_drupal_ui/src/Tests/d6/MigrateUpgrade6Test.php
rename to core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
index 1513bf57e7..0578675c05 100644
--- a/core/modules/migrate_drupal_ui/src/Tests/d6/MigrateUpgrade6Test.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
@@ -1,8 +1,8 @@
 <?php
 
-namespace Drupal\migrate_drupal_ui\Tests\d6;
+namespace Drupal\Tests\migrate_drupal_ui\Functional\d6;
 
-use Drupal\migrate_drupal_ui\Tests\MigrateUpgradeTestBase;
+use Drupal\Tests\migrate_drupal_ui\Functional\MigrateUpgradeTestBase;
 use Drupal\user\Entity\User;
 
 /**
@@ -83,7 +83,7 @@ public function testMigrateUpgrade() {
 
     // Ensure migrated users can log in.
     $user = User::load(2);
-    $user->pass_raw = 'john.doe_pass';
+    $user->passRaw = 'john.doe_pass';
     $this->drupalLogin($user);
   }
 
diff --git a/core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/html-1.txt b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/html-1.txt
similarity index 100%
rename from core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/html-1.txt
rename to core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/html-1.txt
diff --git a/core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/image-1.png b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/image-1.png
similarity index 100%
rename from core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/image-1.png
rename to core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/image-1.png
diff --git a/core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/image-2.jpg b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/image-2.jpg
similarity index 100%
rename from core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/image-2.jpg
rename to core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/image-2.jpg
diff --git a/core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/image-test.gif b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/image-test.gif
similarity index 100%
rename from core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/image-test.gif
rename to core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/image-test.gif
diff --git a/core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/image-test.jpg b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/image-test.jpg
similarity index 100%
rename from core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/image-test.jpg
rename to core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/image-test.jpg
diff --git a/core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/image-test.png b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/image-test.png
similarity index 100%
rename from core/modules/migrate_drupal_ui/src/Tests/d6/files/core/modules/simpletest/files/image-test.png
rename to core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/modules/simpletest/files/image-test.png
diff --git a/core/modules/migrate_drupal_ui/src/Tests/d6/files/tmp/some-temp-file.jpg b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/tmp/some-temp-file.jpg
similarity index 100%
rename from core/modules/migrate_drupal_ui/src/Tests/d6/files/tmp/some-temp-file.jpg
rename to core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/tmp/some-temp-file.jpg
diff --git a/core/modules/migrate_drupal_ui/src/Tests/d7/MigrateUpgrade7Test.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
similarity index 92%
rename from core/modules/migrate_drupal_ui/src/Tests/d7/MigrateUpgrade7Test.php
rename to core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
index 94a42e2c95..6bb46a9fec 100644
--- a/core/modules/migrate_drupal_ui/src/Tests/d7/MigrateUpgrade7Test.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
@@ -1,8 +1,8 @@
 <?php
 
-namespace Drupal\migrate_drupal_ui\Tests\d7;
+namespace Drupal\Tests\migrate_drupal_ui\Functional\d7;
 
-use Drupal\migrate_drupal_ui\Tests\MigrateUpgradeTestBase;
+use Drupal\Tests\migrate_drupal_ui\Functional\MigrateUpgradeTestBase;
 use Drupal\user\Entity\User;
 
 /**
@@ -84,7 +84,7 @@ public function testMigrateUpgrade() {
 
     // Ensure migrated users can log in.
     $user = User::load(2);
-    $user->pass_raw = 'a password';
+    $user->passRaw = 'a password';
     $this->drupalLogin($user);
   }
 
diff --git a/core/modules/migrate_drupal_ui/src/Tests/d7/files/sites/default/files/cube.jpeg b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/files/sites/default/files/cube.jpeg
similarity index 100%
rename from core/modules/migrate_drupal_ui/src/Tests/d7/files/sites/default/files/cube.jpeg
rename to core/modules/migrate_drupal_ui/tests/src/Functional/d7/files/sites/default/files/cube.jpeg
diff --git a/core/modules/migrate_drupal_ui/src/Tests/d7/files/sites/default/files/ds9.txt b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/files/sites/default/files/ds9.txt
similarity index 100%
rename from core/modules/migrate_drupal_ui/src/Tests/d7/files/sites/default/files/ds9.txt
rename to core/modules/migrate_drupal_ui/tests/src/Functional/d7/files/sites/default/files/ds9.txt