diff -u b/core/includes/theme.inc b/core/includes/theme.inc
--- b/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1763,7 +1763,7 @@
       'render element' => 'element',
     ),
     'status_messages' => array(
-      'variables' => array('status_headings' => array(), 'message_list' => NULL),
+      'variables' => ['status_headings' => [], 'message_list' => NULL],
       'template' => 'status-messages',
     ),
     'links' => array(
diff -u b/core/modules/system/templates/install-page.html.twig b/core/modules/system/templates/install-page.html.twig
--- b/core/modules/system/templates/install-page.html.twig
+++ b/core/modules/system/templates/install-page.html.twig
@@ -31,7 +31,6 @@
         <h1>{{ title }}</h1>
       {% endif %}
       {{ page.messages }}
-      {{ messages }}
       {{ page.content }}
     </main>
 
diff -u b/core/modules/system/templates/maintenance-page.html.twig b/core/modules/system/templates/maintenance-page.html.twig
--- b/core/modules/system/templates/maintenance-page.html.twig
+++ b/core/modules/system/templates/maintenance-page.html.twig
@@ -42,7 +42,6 @@
     {% endif %}
 
     {{ page.messages }}
-    {{ messages }}
 
     {{ page.content }}
   </main>
diff -u b/core/modules/system/templates/page.html.twig b/core/modules/system/templates/page.html.twig
--- b/core/modules/system/templates/page.html.twig
+++ b/core/modules/system/templates/page.html.twig
@@ -98,7 +98,6 @@
   {{ page.breadcrumb }}
 
   {{ page.messages }}
-  {{ messages }}
 
   {{ page.help }}
 
diff -u b/core/themes/bartik/templates/page.html.twig b/core/themes/bartik/templates/page.html.twig
--- b/core/themes/bartik/templates/page.html.twig
+++ b/core/themes/bartik/templates/page.html.twig
@@ -38,7 +38,6 @@
  * - title: The page title, for use in the actual content.
  * - title_suffix: Additional output populated by modules, intended to be
  *   displayed after the main title tag that appears in the template.
- * - messages: Status and error messages. Should be displayed prominently.
  * - tabs: Tabs linking to any sub-pages beneath the current page (e.g., the
  *   view and edit tabs when displaying a node).
  * - action_links: Actions local to the page, such as "Add menu" on the menu
diff -u b/core/themes/classy/templates/system/page.html.twig b/core/themes/classy/templates/system/page.html.twig
--- b/core/themes/classy/templates/system/page.html.twig
+++ b/core/themes/classy/templates/system/page.html.twig
@@ -31,7 +31,6 @@
  * - title: The page title, for use in the actual content.
  * - title_suffix: Additional output populated by modules, intended to be
  *   displayed after the main title tag that appears in the template.
- * - messages: Status and error messages. Should be displayed prominently.
  * - tabs: Tabs linking to any sub-pages beneath the current page (e.g., the
  *   view and edit tabs when displaying a node).
  * - action_links: Actions local to the page, such as "Add menu" on the menu
@@ -97,7 +96,6 @@
   {{ page.breadcrumb }}
 
   {{ page.messages }}
-  {{ messages }}
 
   {{ page.help }}
 
diff -u b/core/themes/seven/templates/install-page.html.twig b/core/themes/seven/templates/install-page.html.twig
--- b/core/themes/seven/templates/install-page.html.twig
+++ b/core/themes/seven/templates/install-page.html.twig
@@ -28,7 +28,6 @@
       <h1>{{ title }}</h1>
     {% endif %}
     {{ page.messages }}
-    {{ messages }}
     {{ page.content }}
   </main>
 
diff -u b/core/themes/seven/templates/maintenance-page.html.twig b/core/themes/seven/templates/maintenance-page.html.twig
--- b/core/themes/seven/templates/maintenance-page.html.twig
+++ b/core/themes/seven/templates/maintenance-page.html.twig
@@ -28,8 +28,6 @@
       <h1>{{ title }}</h1>
     {% endif %}
     {{ page.messages }}
-    {{ messages }}
-    {{ page.content }}
   </main>
 
   {% if page.page_bottom %}
diff -u b/core/themes/seven/templates/page.html.twig b/core/themes/seven/templates/page.html.twig
--- b/core/themes/seven/templates/page.html.twig
+++ b/core/themes/seven/templates/page.html.twig
@@ -32,7 +32,6 @@
  * - title: The page title, for use in the actual content.
  * - title_suffix: Additional output populated by modules, intended to be
  *   displayed after the main title tag that appears in the template.
- * - messages: Status and error messages. Should be displayed prominently.
  * - tabs: Tabs linking to any sub-pages beneath the current page (e.g., the
  *   view and edit tabs when displaying a node).
  * - action_links: Actions local to the page, such as "Add menu" on the menu
only in patch2:
unchanged:
--- a/core/modules/action/src/Tests/BulkFormTest.php
+++ b/core/modules/action/src/Tests/BulkFormTest.php
@@ -23,7 +23,15 @@ class BulkFormTest extends WebTestBase {
    *
    * @var array
    */
-  public static $modules = array('node', 'action_bulk_test');
+  public static $modules = array('node', 'action_bulk_test', 'block');
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->drupalPlaceBlock('system_page_messages_block');
+  }
 
   /**
    * Tests the bulk form.
only in patch2:
unchanged:
--- a/core/modules/action/src/Tests/ConfigurationTest.php
+++ b/core/modules/action/src/Tests/ConfigurationTest.php
@@ -23,7 +23,15 @@ class ConfigurationTest extends WebTestBase {
    *
    * @var array
    */
-  public static $modules = array('action');
+  public static $modules = array('action', 'block');
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->drupalPlaceBlock('system_page_messages_block');
+  }
 
   /**
    * Tests configuration of advanced actions through administration interface.
only in patch2:
unchanged:
--- a/core/modules/aggregator/src/Tests/AggregatorTestBase.php
+++ b/core/modules/aggregator/src/Tests/AggregatorTestBase.php
@@ -21,11 +21,16 @@
    *
    * @var array
    */
-  public static $modules = array('node', 'aggregator', 'aggregator_test', 'views');
+  public static $modules = ['node', 'aggregator', 'aggregator_test', 'views', 'block'];
 
+  /**
+   * {@inheritdoc}
+   */
   protected function setUp() {
     parent::setUp();
 
+    $this->drupalPlaceBlock('system_page_messages_block');
+
     // Create an Article node type.
     if ($this->profile != 'standard') {
       $this->drupalCreateContentType(array('type' => 'article', 'name' => 'Article'));
only in patch2:
unchanged:
--- a/core/modules/ban/src/Tests/IpAddressBlockingTest.php
+++ b/core/modules/ban/src/Tests/IpAddressBlockingTest.php
@@ -21,7 +21,15 @@ class IpAddressBlockingTest extends WebTestBase {
    *
    * @var array
    */
-  public static $modules = array('ban');
+  public static $modules = array('ban', 'block');
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->drupalPlaceBlock('system_page_messages_block');
+  }
 
   /**
    * Tests various user input to confirm correct validation and saving of data.
only in patch2:
unchanged:
--- a/core/modules/block/src/Tests/BlockInvalidRegionTest.php
+++ b/core/modules/block/src/Tests/BlockInvalidRegionTest.php
@@ -25,8 +25,12 @@ class BlockInvalidRegionTest extends WebTestBase {
    */
   public static $modules = array('block', 'block_test');
 
+  /**
+   * {@inheritdoc}
+   */
   protected function setUp() {
     parent::setUp();
+    $this->drupalPlaceBlock('system_page_messages_block');
     // Create an admin user.
     $admin_user = $this->drupalCreateUser(array(
       'administer site configuration',
only in patch2:
unchanged:
--- a/core/modules/block/src/Tests/BlockLanguageCacheTest.php
+++ b/core/modules/block/src/Tests/BlockLanguageCacheTest.php
@@ -32,9 +32,14 @@ class BlockLanguageCacheTest extends WebTestBase {
    */
   protected $langcodes = array();
 
+  /**
+   * {@inheritdoc}
+   */
   protected function setUp() {
     parent::setUp();
 
+    $this->drupalPlaceBlock('system_page_messages_block');
+
     // Create test languages.
     $this->langcodes = array(ConfigurableLanguage::load('en'));
     for ($i = 1; $i < 3; ++$i) {
only in patch2:
unchanged:
--- a/core/modules/block/src/Tests/BlockTestBase.php
+++ b/core/modules/block/src/Tests/BlockTestBase.php
@@ -35,9 +35,14 @@
    */
   protected $adminUser;
 
+  /**
+   * {@inheritdoc}
+   */
   protected function setUp() {
     parent::setUp();
 
+    $this->drupalPlaceBlock('system_page_messages_block');
+
     // Use the test page as the front page.
     $this->config('system.site')->set('page.front', 'test-page')->save();
 
only in patch2:
unchanged:
--- a/core/modules/block_content/src/Tests/BlockContentCreationTest.php
+++ b/core/modules/block_content/src/Tests/BlockContentCreationTest.php
@@ -25,13 +25,14 @@ class BlockContentCreationTest extends BlockContentTestBase {
    *
    * @var array
    */
-  public static $modules = array('block_content_test', 'dblog', 'field_ui');
+  public static $modules = ['block', 'block_content_test', 'dblog', 'field_ui'];
 
   /**
-   * Sets the test up.
+   * {@inheritdoc}
    */
   protected function setUp() {
     parent::setUp();
+    $this->drupalPlaceBlock('system_page_messages_block');
     $this->drupalLogin($this->adminUser);
   }
 
only in patch2:
unchanged:
--- a/core/modules/book/src/Tests/BookTest.php
+++ b/core/modules/book/src/Tests/BookTest.php
@@ -22,7 +22,7 @@ class BookTest extends WebTestBase {
    *
    * @var array
    */
-  public static $modules = array('book', 'block', 'node_access_test');
+  public static $modules = ['book', 'block', 'node_access_test'];
 
   /**
    * A book node.
@@ -65,6 +65,7 @@ class BookTest extends WebTestBase {
   protected function setUp() {
     parent::setUp();
     $this->drupalPlaceBlock('system_breadcrumb_block');
+    $this->drupalPlaceBlock('system_page_messages_block');
 
     // node_access_test requires a node_access_rebuild().
     node_access_rebuild();