diff -u b/core/modules/block/src/BlockForm.php b/core/modules/block/src/BlockForm.php
--- b/core/modules/block/src/BlockForm.php
+++ b/core/modules/block/src/BlockForm.php
@@ -12,7 +12,7 @@
 use Drupal\Core\Entity\EntityForm;
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Executable\ExecutableManagerInterface;
-use Drupal\Core\Extension\ThemeHandler;
+use Drupal\Core\Extension\ThemeHandlerInterface;
 use Drupal\Core\Form\FormState;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Language\LanguageManagerInterface;
@@ -78,10 +78,10 @@
    *   The EventDispatcher for gathering administrative contexts.
    * @param \Drupal\Core\Language\LanguageManagerInterface $language
    *   The language manager.
-   * @param \Drupal\Core\Extension\ThemeHandler $theme_handler
+   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
    * The theme handler.
    */
-  public function __construct(EntityManagerInterface $entity_manager, ExecutableManagerInterface $manager, EventDispatcherInterface $dispatcher, LanguageManagerInterface $language, ThemeHandler $theme_handler) {
+  public function __construct(EntityManagerInterface $entity_manager, ExecutableManagerInterface $manager, EventDispatcherInterface $dispatcher, LanguageManagerInterface $language, ThemeHandlerInterface $theme_handler) {
     $this->storage = $entity_manager->getStorage('block');
     $this->manager = $manager;
     $this->dispatcher = $dispatcher;
diff -u b/core/modules/block_content/src/Controller/BlockContentController.php b/core/modules/block_content/src/Controller/BlockContentController.php
--- b/core/modules/block_content/src/Controller/BlockContentController.php
+++ b/core/modules/block_content/src/Controller/BlockContentController.php
@@ -11,7 +11,7 @@
 use Drupal\Core\Controller\ControllerBase;
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\block_content\BlockContentTypeInterface;
-use Drupal\Core\Extension\ThemeHandler;
+use Drupal\Core\Extension\ThemeHandlerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\HttpFoundation\Request;
 
@@ -34,7 +34,7 @@
   /**
    * The theme handler.
    *
-   * @var \Drupal\Core\Extension\ThemeHandler
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
    */
   protected $themeHandler;
 
@@ -57,10 +57,10 @@
    *   The custom block storage.
    * @param \Drupal\Core\Entity\EntityStorageInterface $block_content_type_storage
    *   The custom block type storage.
-   * @param \Drupal\Core\Extension\ThemeHandler $theme_handler
+   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
    * The theme handler.
    */
-  public function __construct(EntityStorageInterface $block_content_storage, EntityStorageInterface $block_content_type_storage, ThemeHandler $theme_handler) {
+  public function __construct(EntityStorageInterface $block_content_storage, EntityStorageInterface $block_content_type_storage, ThemeHandlerInterface $theme_handler) {
     $this->blockContentStorage = $block_content_storage;
     $this->blockContentTypeStorage = $block_content_type_storage;
     $this->themeHandler = $theme_handler;
diff -u b/core/modules/system/src/Form/ThemeSettingsForm.php b/core/modules/system/src/Form/ThemeSettingsForm.php
--- b/core/modules/system/src/Form/ThemeSettingsForm.php
+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\system\Form;
 
-use Drupal\Core\Extension\ThemeHandler;
+use Drupal\Core\Extension\ThemeHandlerInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Render\Element;
 use Drupal\Core\StreamWrapper\PublicStream;
@@ -33,7 +33,7 @@
   /**
    * The theme handler.
    *
-   * @var \Drupal\Core\Extension\ThemeHandler
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
    */
   protected $themeHandler;
 
@@ -44,10 +44,10 @@
    *   The factory for configuration objects.
    * @param \Drupal\Core\Extension\ModuleHandlerInterface
    *   The module handler instance to use.
-   * @param \Drupal\Core\Extension\ThemeHandler $theme_handler
+   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
    * The theme handler.
    */
-  public function __construct(ConfigFactoryInterface $config_factory, ModuleHandlerInterface $module_handler, ThemeHandler $theme_handler) {
+  public function __construct(ConfigFactoryInterface $config_factory, ModuleHandlerInterface $module_handler, ThemeHandlerInterface $theme_handler) {
     parent::__construct($config_factory);
 
     $this->moduleHandler = $module_handler;
diff -u b/core/modules/views_ui/src/Form/BasicSettingsForm.php b/core/modules/views_ui/src/Form/BasicSettingsForm.php
--- b/core/modules/views_ui/src/Form/BasicSettingsForm.php
+++ b/core/modules/views_ui/src/Form/BasicSettingsForm.php
@@ -8,7 +8,7 @@
 namespace Drupal\views_ui\Form;
 
 use Drupal\Core\Config\ConfigFactoryInterface;
-use Drupal\Core\Extension\ThemeHandler;
+use Drupal\Core\Extension\ThemeHandlerInterface;
 use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\Form\FormStateInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
@@ -21,7 +21,7 @@
   /**
    * The theme handler.
    *
-   * @var \Drupal\Core\Extension\ThemeHandler
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
    */
   protected $themeHandler;
 
@@ -30,10 +30,10 @@
    *
    * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
    *   The factory for configuration objects.
-   * @param \Drupal\Core\Extension\ThemeHandler $theme_handler
+   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
    *   The theme handler.
    */
-  public function __construct(ConfigFactoryInterface $config_factory, ThemeHandler $theme_handler) {
+  public function __construct(ConfigFactoryInterface $config_factory, ThemeHandlerInterface $theme_handler) {
     parent::__construct($config_factory);
 
     $this->themeHandler = $theme_handler;
only in patch2:
unchanged:
--- a/core/modules/block/tests/src/Unit/BlockFormTest.php
+++ b/core/modules/block/tests/src/Unit/BlockFormTest.php
@@ -44,6 +44,14 @@ class BlockFormTest extends UnitTestCase {
    */
   protected $language;
 
+
+  /**
+   * The theme handler.
+   *
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $themeHandler;
+
   /**
    * The entity manager.
    *
@@ -63,6 +71,7 @@ protected function setUp() {
 
     $this->entityManager = $this->getMock('Drupal\Core\Entity\EntityManagerInterface');
     $this->storage = $this->getMock('Drupal\Core\Config\Entity\ConfigEntityStorageInterface');
+    $this->themeHandler = $this->getMock('Drupal\Core\Extension\ThemeHandlerInterface');
     $this->entityManager->expects($this->any())
       ->method('getStorage')
       ->will($this->returnValue($this->storage));
@@ -95,7 +104,7 @@ public function testGetUniqueMachineName() {
       ->method('getQuery')
       ->will($this->returnValue($query));
 
-    $block_form_controller = new BlockForm($this->entityManager, $this->conditionManager, $this->dispatcher, $this->language);
+    $block_form_controller = new BlockForm($this->entityManager, $this->conditionManager, $this->dispatcher, $this->language, $this->themeHandler);
 
     // Ensure that the block with just one other instance gets the next available
     // name suggestion.