diff --git a/core/lib/Drupal/Component/Utility/SafeMarkup.php b/core/lib/Drupal/Component/Utility/SafeMarkup.php
index 69d12fe..598b5f1 100644
--- a/core/lib/Drupal/Component/Utility/SafeMarkup.php
+++ b/core/lib/Drupal/Component/Utility/SafeMarkup.php
@@ -91,7 +91,7 @@ public static function set($string, $strategy = 'html') {
    *   TRUE if the string has been marked secure, FALSE otherwise.
    */
   public static function isSafe($string, $strategy = 'html') {
-    return isset(static::$safeStrings[(string) $string][$strategy]) ||
+    return ($string instanceof \Twig_Markup) || isset(static::$safeStrings[(string) $string][$strategy]) ||
       isset(static::$safeStrings[(string) $string]['all']);
   }
 
diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index 6850fcb..c991cf0 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -385,7 +385,7 @@ protected function theme($hook, $variables = array()) {
       $output = $render_function($template_file, $variables);
     }
 
-    return (string) $output;
+    return ($output instanceof \Twig_Markup) ? $output : (string) $output;
   }
 
   /**
diff --git a/core/modules/simpletest/src/AssertContentTrait.php b/core/modules/simpletest/src/AssertContentTrait.php
index 240aea9..8232cb0 100644
--- a/core/modules/simpletest/src/AssertContentTrait.php
+++ b/core/modules/simpletest/src/AssertContentTrait.php
@@ -808,7 +808,7 @@ protected function assertNoTitle($title, $message = '', $group = 'Other') {
    *   TRUE on pass, FALSE on fail.
    */
   protected function assertThemeOutput($callback, array $variables = array(), $expected = '', $message = '', $group = 'Other') {
-    $output = \Drupal::theme()->render($callback, $variables);
+    $output = (string) \Drupal::theme()->render($callback, $variables);
     $this->verbose(
       '<hr />' . 'Result:' . '<pre>' . SafeMarkup::checkPlain(var_export($output, TRUE)) . '</pre>'
       . '<hr />' . 'Expected:' . '<pre>' . SafeMarkup::checkPlain(var_export($expected, TRUE)) . '</pre>'
diff --git a/core/modules/system/templates/dropbutton-wrapper.html.twig b/core/modules/system/templates/dropbutton-wrapper.html.twig
index ca0ff7e..d4c8d90 100644
--- a/core/modules/system/templates/dropbutton-wrapper.html.twig
+++ b/core/modules/system/templates/dropbutton-wrapper.html.twig
@@ -12,7 +12,7 @@
  * @ingroup themeable
  */
 #}
-{% if children %}
+{% if children|length %}
   {% spaceless %}
     <div class="dropbutton-wrapper">
       <div class="dropbutton-widget">
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index 9f8bc4a..b7df114 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -103,7 +103,8 @@ function twig_render_template($template_file, array $variables) {
     $output['debug_info']   .= "\n<!-- BEGIN OUTPUT from '" . SafeMarkup::checkPlain($template_file) . "' -->\n";
     $output['debug_suffix'] .= "\n<!-- END OUTPUT from '" . SafeMarkup::checkPlain($template_file) . "' -->\n\n";
   }
-  return SafeMarkup::set(implode('', $output));
+  // This output has already been rendered and is therefore considered safe.
+  return new \Twig_Markup(implode('', $output), 'UTF-8');
 }
 
 /**