diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 87ef625..37f04f9 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -2197,9 +2197,6 @@ function template_preprocess_page(&$variables) {
   $language_interface = language(Language::TYPE_INTERFACE);
   $site_config = \Drupal::config('system.site');
 
-  $page_object = $variables['page']['#page'];
-  $attributes = $page_object->getBodyAttributes();
-
   // Move some variables to the top level for themer convenience and template cleanliness.
   $variables['show_messages'] = $variables['page']['#show_messages'];
 
diff --git a/core/modules/aggregator/aggregator.routing.yml b/core/modules/aggregator/aggregator.routing.yml
index 33ac34d..1f3464d 100644
--- a/core/modules/aggregator/aggregator.routing.yml
+++ b/core/modules/aggregator/aggregator.routing.yml
@@ -57,7 +57,7 @@ aggregator.feed_edit:
 aggregator.feed_refresh:
   path: '/admin/config/services/aggregator/update/{aggregator_feed}'
   defaults:
-    _controller: '\Drupal\aggregator\Controller\AggregatorController::feedRefresh'
+    _content: '\Drupal\aggregator\Controller\AggregatorController::feedRefresh'
     _title: 'Update items'
   requirements:
     _permission: 'administer news feeds'
diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index bd50676..b211675 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -1,6 +1,7 @@
 <?php
 
 use Drupal\Core\Database\Database;
+use Drupal\Core\Page\HtmlPage;
 use Drupal\simpletest\TestBase;
 use Symfony\Component\Process\PhpExecutableFinder;
 
@@ -877,3 +878,20 @@ function simpletest_phpunit_testcase_to_row($test_id, \SimpleXMLElement $testcas
   );
   return $record;
 }
+
+/**
+ * Prepares variables for the page template.
+ */
+function simpletest_preprocess_page(&$variables) {
+  // If controllers accidentally use _controller instead of _content, but do not
+  // return a response object or an HTML page object the site still renders
+  // properly but using the page object in template_preprocess_page() will fail.
+  // We therefore throw an exception on the test site in this case so that every
+  // call to WebTestBase::drupalGet() automatically benefits from this coverage.
+  if (drupal_valid_test_ua()) {
+    $page_object = $variables['page']['#page'];
+    if (!($page_object instanceof HtmlPage)) {
+      throw new \Exception('The page object is not an instance of \Drupal\Core\Page\HtmlPage');
+    }
+  }
+}
diff --git a/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
index 82ae136..aec3355 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
@@ -60,7 +60,7 @@ function testPageCacheTags() {
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS');
     $this->drupalGet($path);
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT');
-    Cache::deleteTags($tags);
+    Cache::invalidateTags($tags);
     $this->drupalGet($path);
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS');
   }