Issue #2853163 by douggreen: ThemeInstaller is missing protected variable...

Issue #2853163 by douggreen: ThemeInstaller is missing protected variable declaration for injected themeHandler

core/lib/Drupal/Core/Extension/ThemeInstaller.php

@@ -16,6 +16,11 @@
  */
 class ThemeInstaller implements ThemeInstallerInterface {
 
+  /**
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
+   */
+  protected $themeHandler;
+
   /**
    * @var \Drupal\Core\Config\ConfigFactoryInterface
    */
@@ -56,7 +61,6 @@ class ThemeInstaller implements ThemeInstallerInterface {
    */
   protected $logger;
 
-
   /**
    * Constructs a new ThemeInstaller.
    *