diff --git a/core/core.services.yml b/core/core.services.yml
index 6049d3b..090b692 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -1102,7 +1102,7 @@ services:
     arguments: ['@state', '@current_user']
   maintenance_mode_subscriber:
     class: Drupal\Core\EventSubscriber\MaintenanceModeSubscriber
-    arguments: ['@maintenance_mode', '@config.factory', '@string_translation', '@url_generator', '@current_user', '@bare_html_page_renderer']
+    arguments: ['@maintenance_mode', '@config.factory', '@string_translation', '@url_generator', '@current_user', '@bare_html_page_renderer', '@messenger']
     tags:
       - { name: event_subscriber }
   path_subscriber:
@@ -1607,3 +1607,6 @@ services:
     arguments: ['@current_user', '@path.current', '@path.matcher', '@language_manager']
     tags:
       - { name: event_subscriber }
+  messenger:
+    class: Drupal\Core\Messenger\SessionMessenger
+    arguments: ['@session_manager', '@page_cache_kill_switch']
diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 2294d47..8d901a8 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -9,8 +9,6 @@
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Logger\RfcLogLevel;
-use Drupal\Core\Render\Markup;
-use Drupal\Component\Render\MarkupInterface;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\Site\Settings;
 use Drupal\Core\Utility\Error;
@@ -436,30 +434,15 @@ function watchdog_exception($type, Exception $exception, $message = NULL, $varia
  *
  * @see drupal_get_messages()
  * @see status-messages.html.twig
+ *
+ * @deprecated Deprecated as of Drupal 8.2.
+ *   Use \Drupal::service('messenger')->addMessage() instead.
  */
 function drupal_set_message($message = NULL, $type = 'status', $repeat = FALSE) {
-  if (isset($message)) {
-    if (!isset($_SESSION['messages'][$type])) {
-      $_SESSION['messages'][$type] = array();
-    }
-
-    // Convert strings which are safe to the simplest Markup objects.
-    if (!($message instanceof Markup) && $message instanceof MarkupInterface) {
-      $message = Markup::create((string) $message);
-    }
-
-    // Do not use strict type checking so that equivalent string and
-    // MarkupInterface objects are detected.
-    if ($repeat || !in_array($message, $_SESSION['messages'][$type])) {
-      $_SESSION['messages'][$type][] = $message;
-    }
-
-    // Mark this page as being uncacheable.
-    \Drupal::service('page_cache_kill_switch')->trigger();
-  }
-
-  // Messages not set when DB connection fails.
-  return isset($_SESSION['messages']) ? $_SESSION['messages'] : NULL;
+  /* @var \Drupal\Core\Messenger\MessengerInterface $messenger */
+  $messenger = \Drupal::service('messenger');
+  $messenger->addMessage($message, $type, $repeat);
+  return $messenger->getMessages();
 }
 
 /**
@@ -486,12 +469,24 @@ function drupal_set_message($message = NULL, $type = 'status', $repeat = FALSE)
  *
  * @see drupal_set_message()
  * @see status-messages.html.twig
+ *
+ * @deprecated Deprecated as of Drupal 8.2.
+ *   Use \Drupal::service('messenger')->getMessages() or
+ *   \Drupal::service('messenger')->getMessagesByType() instead.
  */
 function drupal_get_messages($type = NULL, $clear_queue = TRUE) {
-  if ($messages = drupal_set_message()) {
+  // Workaround for code that can not check if the service exists.
+  if (!\Drupal::hasService('messenger')) {
+    return [];
+  }
+
+  /** @var \Drupal\Core\Messenger\MessengerInterface $messenger */
+  $messenger = \Drupal::service('messenger');
+
+  if ($messages = $messenger->getMessages()) {
     if ($type) {
       if ($clear_queue) {
-        unset($_SESSION['messages'][$type]);
+        $messenger->deleteMessagesByType($type);
       }
       if (isset($messages[$type])) {
         return array($type => $messages[$type]);
@@ -499,7 +494,7 @@ function drupal_get_messages($type = NULL, $clear_queue = TRUE) {
     }
     else {
       if ($clear_queue) {
-        unset($_SESSION['messages']);
+        $messenger->deleteMessages();
       }
       return $messages;
     }
diff --git a/core/lib/Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php
index 873996b..3e9fac5 100644
--- a/core/lib/Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php
@@ -6,6 +6,7 @@
 use Drupal\Component\Utility\Xss;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Render\BareHtmlPageRendererInterface;
+use Drupal\Core\Messenger\MessengerInterface;
 use Drupal\Core\Routing\RouteMatch;
 use Drupal\Core\Routing\UrlGeneratorInterface;
 use Drupal\Core\Session\AccountInterface;
@@ -59,6 +60,13 @@ class MaintenanceModeSubscriber implements EventSubscriberInterface {
   protected $bareHtmlPageRenderer;
 
   /**
+   * The messenger.
+   *
+   * @var \Drupal\Core\Messenger\MessengerInterface
+   */
+  protected $messenger;
+
+  /**
    * Constructs a new MaintenanceModeSubscriber.
    *
    * @param \Drupal\Core\Site\MaintenanceModeInterface $maintenance_mode
@@ -73,14 +81,17 @@ class MaintenanceModeSubscriber implements EventSubscriberInterface {
    *   The current user.
    * @param \Drupal\Core\Render\BareHtmlPageRendererInterface $bare_html_page_renderer
    *   The bare HTML page renderer.
+   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
+   *   The messenger.
    */
-  public function __construct(MaintenanceModeInterface $maintenance_mode, ConfigFactoryInterface $config_factory, TranslationInterface $translation, UrlGeneratorInterface $url_generator, AccountInterface $account, BareHtmlPageRendererInterface $bare_html_page_renderer) {
+  public function __construct(MaintenanceModeInterface $maintenance_mode, ConfigFactoryInterface $config_factory, TranslationInterface $translation, UrlGeneratorInterface $url_generator, AccountInterface $account, BareHtmlPageRendererInterface $bare_html_page_renderer, MessengerInterface $messenger) {
     $this->maintenanceMode = $maintenance_mode;
     $this->config = $config_factory;
     $this->stringTranslation = $translation;
     $this->urlGenerator = $url_generator;
     $this->account = $account;
     $this->bareHtmlPageRenderer = $bare_html_page_renderer;
+    $this->messenger = $messenger;
   }
 
   /**
@@ -111,10 +122,10 @@ public function onKernelRequestMaintenance(GetResponseEvent $event) {
         // settings page.
         if ($route_match->getRouteName() != 'system.site_maintenance_mode') {
           if ($this->account->hasPermission('administer site configuration')) {
-            $this->drupalSetMessage($this->t('Operating in maintenance mode. <a href=":url">Go online.</a>', array(':url' => $this->urlGenerator->generate('system.site_maintenance_mode'))), 'status', FALSE);
+            $this->messenger->addMessage($this->t('Operating in maintenance mode. <a href="@url">Go online.</a>', array('@url' => $this->urlGenerator->generate('system.site_maintenance_mode'))), 'status', FALSE);
           }
           else {
-            $this->drupalSetMessage($this->t('Operating in maintenance mode.'), 'status', FALSE);
+            $this->messenger->addMessage($this->t('Operating in maintenance mode.'), 'status', FALSE);
           }
         }
       }
@@ -122,13 +133,6 @@ public function onKernelRequestMaintenance(GetResponseEvent $event) {
   }
 
   /**
-   * Wraps the drupal_set_message function.
-   */
-  protected function drupalSetMessage($message = NULL, $type = 'status', $repeat = FALSE) {
-    return drupal_set_message($message, $type, $repeat);
-  }
-
-  /**
    * {@inheritdoc}
    */
   public static function getSubscribedEvents() {
diff --git a/core/lib/Drupal/Core/Installer/InstallerServiceProvider.php b/core/lib/Drupal/Core/Installer/InstallerServiceProvider.php
index 5603e4c..e2670b0 100644
--- a/core/lib/Drupal/Core/Installer/InstallerServiceProvider.php
+++ b/core/lib/Drupal/Core/Installer/InstallerServiceProvider.php
@@ -34,6 +34,9 @@ public function register(ContainerBuilder $container) {
       ->register('keyvalue', 'Drupal\Core\KeyValueStore\KeyValueMemoryFactory');
     $container
       ->register('keyvalue.expirable', 'Drupal\Core\KeyValueStore\KeyValueNullExpirableFactory');
+    $definition = $container->getDefinition('messenger');
+    $definition->setClass(StaticMessenger::class);
+    $definition->setArguments([new Reference('page_cache_kill_switch')]);
 
     // Replace services with no-op implementations.
     $container
diff --git a/core/lib/Drupal/Core/Messenger/MessengerInterface.php b/core/lib/Drupal/Core/Messenger/MessengerInterface.php
new file mode 100644
index 0000000..d9b7547
--- /dev/null
+++ b/core/lib/Drupal/Core/Messenger/MessengerInterface.php
@@ -0,0 +1,79 @@
+<?php
+
+namespace Drupal\Core\Messenger;
+
+/**
+ * Stores messages send out to the user on the page.
+ */
+interface MessengerInterface {
+
+  /**
+   * A status message.
+   */
+  const STATUS = 'status';
+
+  /**
+   * A warning.
+   */
+  const WARNING = 'warning';
+
+  /**
+   * An error.
+   */
+  const ERROR = 'error';
+
+  /**
+   * Adds a new message to the queue.
+   *
+   * @param string $message
+   *   (optional) The translated message to be displayed to the user. For
+   *   consistency with other messages, it should begin with a capital letter
+   *   and end with a period.
+   * @param string $type
+   *   (optional) The message's type. Either self::STATUS, self::WARNING, or
+   *   self::ERROR.
+   * @param bool $repeat
+   *   (optional) If this is FALSE and the message is already set, then the
+   *   message won't be repeated. Defaults to FALSE.
+   *
+   * @return $this
+   */
+  public function addMessage($message, $type = self::STATUS, $repeat = FALSE);
+
+  /**
+   * Gets all messages.
+   *
+   * @return array[]
+   *   Keys are message types and values are indexed arrays of messages. Message
+   *   types are either self::STATUS, self::WARNING, or self::ERROR.
+   */
+  public function getMessages();
+
+  /**
+   * Gets all messages of a certain type.
+   *
+   * @param string $type
+   *   The messages' type. Either self::STATUS, self::WARNING, or self::ERROR.
+   *
+   * @return string[]
+   */
+  public function getMessagesByType($type);
+
+  /**
+   * Deletes all messages.
+   *
+   * @return $this
+   */
+  public function deleteMessages();
+
+  /**
+   * Deletes all messages of a certain type.
+   *
+   * @param string $type
+   *   The messages' type. Either self::STATUS, self::WARNING, or self::ERROR.
+   *
+   * @return $this
+   */
+  public function deleteMessagesByType($type);
+
+}
diff --git a/core/lib/Drupal/Core/Messenger/SessionMessenger.php b/core/lib/Drupal/Core/Messenger/SessionMessenger.php
new file mode 100644
index 0000000..570df10
--- /dev/null
+++ b/core/lib/Drupal/Core/Messenger/SessionMessenger.php
@@ -0,0 +1,103 @@
+<?php
+
+namespace Drupal\Core\Messenger;
+
+use Drupal\Component\Render\MarkupInterface;
+use Drupal\Core\PageCache\ResponsePolicy\KillSwitch;
+use Drupal\Core\Render\Markup;
+use Drupal\Core\Session\SessionManagerInterface;
+
+/**
+ * Provides a session-based messenger.
+ */
+class SessionMessenger implements MessengerInterface {
+
+  /**
+   * The page caching kill switch.
+   *
+   * @var \Drupal\Core\PageCache\ResponsePolicy\KillSwitch
+   */
+  protected $pageCacheKillSwitch;
+
+  /**
+   * The session manager.
+   *
+   * @var \Drupal\Core\Session\SessionManagerInterface
+   */
+  protected $sessionManager;
+
+  /**
+   * Constructs a new instance.
+   *
+   * @param \Drupal\Core\Session\SessionManagerInterface
+   * @param \Drupal\Core\PageCache\ResponsePolicy\KillSwitch $page_cache_kill_switch
+   *   The page caching kill switch.
+   */
+  public function __construct(SessionManagerInterface $session_manager, KillSwitch $page_cache_kill_switch) {
+    $this->pageCacheKillSwitch = $page_cache_kill_switch;
+    $this->sessionManager = $session_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function addMessage($message, $type = self::STATUS, $repeat = FALSE) {
+    if (!isset($_SESSION['messages'][$type])) {
+      $_SESSION['messages'][$type] = [];
+    }
+
+    // Convert strings which are safe to the simplest Markup objects.
+    if (!($message instanceof Markup) && $message instanceof MarkupInterface) {
+      $message = Markup::create((string) $message);
+    }
+
+    // Do not use strict type checking so that equivalent string and
+    // MarkupInterface objects are detected.
+    if ($repeat || !in_array($message, $_SESSION['messages'][$type])) {
+      $_SESSION['messages'][$type][] = $message;
+      $this->pageCacheKillSwitch->trigger();
+    }
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getMessages() {
+    $messages = isset($_SESSION['messages']) ? $_SESSION['messages'] : [];
+    foreach ($messages as $type => $messages_by_type) {
+      $messages[$type] = $messages_by_type;
+    }
+
+    return $messages;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getMessagesByType($type) {
+    $messages = isset($_SESSION['messages']) && isset($_SESSION['messages'][$type]) ? $_SESSION['messages'][$type] : [];
+
+    return $messages;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deleteMessages() {
+    unset($_SESSION['messages']);
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deleteMessagesByType($type) {
+    unset($_SESSION['messages'][$type]);
+
+    return $this;
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Messenger/StaticMessenger.php b/core/lib/Drupal/Core/Messenger/StaticMessenger.php
new file mode 100644
index 0000000..52f7dcb
--- /dev/null
+++ b/core/lib/Drupal/Core/Messenger/StaticMessenger.php
@@ -0,0 +1,104 @@
+<?php
+
+namespace Drupal\Core\Messenger;
+
+use Drupal\Component\Render\MarkupInterface;
+use Drupal\Core\PageCache\ResponsePolicy\KillSwitch;
+use Drupal\Core\Render\Markup;
+
+/**
+ * Provides a messenger that stores messages for this request only.
+ */
+class StaticMessenger implements MessengerInterface {
+
+  /**
+   * The messages that have been set.
+   *
+   * @var array[]
+   *   Keys are either self::STATUS, self::WARNING, or self::ERROR. Values are
+   *   arrays of arrays with the following keys:
+   *   - message (string): the message.
+   *   - safe (bool): whether the message is marked as safe markup.
+   */
+  protected $messages = [];
+
+  /**
+   * The page caching kill switch.
+   *
+   * @var \Drupal\Core\PageCache\ResponsePolicy\KillSwitch
+   */
+  protected $pageCacheKillSwitch;
+
+  /**
+   * Constructs a new instance.
+   *
+   * @param \Drupal\Core\PageCache\ResponsePolicy\KillSwitch $page_cache_kill_switch
+   *   The page caching kill switch.
+   */
+  public function __construct(KillSwitch $page_cache_kill_switch) {
+    $this->pageCacheKillSwitch = $page_cache_kill_switch;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function addMessage($message, $type = self::STATUS, $repeat = FALSE) {
+    if (!isset($this->messages[$type])) {
+      $this->messages[$type] = [];
+    }
+
+    // Convert strings which are safe to the simplest Markup objects.
+    if (!($message instanceof Markup) && $message instanceof MarkupInterface) {
+      $message = Markup::create((string) $message);
+    }
+
+    // Do not use strict type checking so that equivalent string and
+    // MarkupInterface objects are detected.
+    if ($repeat || !in_array($message, $this->messages[$type])) {
+      $this->messages[$type][] = $message;
+      $this->pageCacheKillSwitch->trigger();
+    }
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getMessages() {
+    $messages = isset($this->messages) ? $this->messages : [];
+    foreach ($messages as $type => $messages_by_type) {
+      $messages[$type] = $messages_by_type;
+    }
+
+    return $messages;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getMessagesByType($type) {
+    $messages = isset($this->messages) && isset($this->messages[$type]) ? $this->messages[$type] : [];
+
+    return $messages;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deleteMessages() {
+    unset($this->messages);
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deleteMessagesByType($type) {
+    unset($this->messages[$type]);
+
+    return $this;
+  }
+
+}
diff --git a/core/tests/Drupal/Tests/Core/Messenger/SessionMessengerTest.php b/core/tests/Drupal/Tests/Core/Messenger/SessionMessengerTest.php
new file mode 100644
index 0000000..951ff10
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Messenger/SessionMessengerTest.php
@@ -0,0 +1,110 @@
+<?php
+
+namespace Drupal\Tests\Core\Messenger;
+
+use Drupal\Core\Messenger\SessionMessenger;
+use Drupal\Core\PageCache\ResponsePolicy\KillSwitch;
+use Drupal\Core\Session\SessionManagerInterface;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Messenger\SessionMessenger
+ * @group messenger
+ */
+class SessionMessengerTest extends UnitTestCase {
+
+  /**
+   * A copy of any existing session data to restore after the test.
+   *
+   * @var array
+   */
+  protected $existingSession;
+
+  /**
+   * The messenger under test.
+   *
+   * @var \Drupal\Core\Messenger\SessionMessenger
+   */
+  protected $messenger;
+
+  /**
+   * The page caching kill switch.
+   *
+   * @var \Drupal\Core\PageCache\ResponsePolicy\KillSwitch|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $pageCacheKillSwitch;
+
+  /**
+   * The session manager.
+   *
+   * @var \Drupal\Core\Session\SessionManagerInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $sessionManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    $this->pageCacheKillSwitch = $this->prophesize(KillSwitch::class);
+    $this->sessionManager = $this->prophesize(SessionManagerInterface::class);
+
+    $this->existingSession = isset($_SESSION) ? $_SESSION : NULL;
+    $_SESSION = [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function tearDown() {
+    if ($this->existingSession !== NULL) {
+      $_SESSION = $this->existingSession;
+    }
+    else {
+      unset($_SESSION);
+    }
+  }
+
+  /**
+   * @covers ::addMessage
+   * @covers ::getMessages
+   * @covers ::getMessagesByType
+   * @covers ::deleteMessages
+   * @covers ::deleteMessagesByType
+   */
+  public function testMessenger() {
+    $this->pageCacheKillSwitch->trigger()->shouldBeCalled();
+
+    $this->sessionManager->start()->shouldBeCalled();
+    $this->messenger = new SessionMessenger($this->sessionManager->reveal(), $this->pageCacheKillSwitch->reveal());
+
+    $message_a = $this->randomMachineName();
+    $type_a = $this->randomMachineName();
+    $message_b = $this->randomMachineName();
+    $type_b = $this->randomMachineName();
+
+    // Test that if there are no messages, the default is an empty array.
+    $this->assertEquals($this->messenger->getMessages(), []);
+
+    // Test that adding a message returns the messenger and that the message can
+    // be retrieved.
+    $this->assertEquals($this->messenger->addMessage($message_a, $type_a), $this->messenger);
+    $this->messenger->addMessage($message_a, $type_a);
+    $this->messenger->addMessage($message_a, $type_a, TRUE);
+    $this->messenger->addMessage($message_b, $type_b, TRUE);
+    $this->assertEquals($this->messenger->getMessages(), [
+      $type_a => [$message_a, $message_a],
+      $type_b => [$message_b],
+    ]);
+
+    // Test deleting messages of a certain type.
+    $this->assertEquals($this->messenger->deleteMessagesByType($type_a), $this->messenger);
+    $this->assertEquals($this->messenger->getMessages(), [
+      $type_b => [$message_b],
+    ]);
+
+    // Test deleting all messages.
+    $this->assertEquals($this->messenger->deleteMessages(), $this->messenger);
+    $this->assertEquals($this->messenger->getMessages(), []);
+  }
+
+}
diff --git a/core/tests/Drupal/Tests/Core/Messenger/StaticMessengerTest.php b/core/tests/Drupal/Tests/Core/Messenger/StaticMessengerTest.php
new file mode 100644
index 0000000..d315895
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Messenger/StaticMessengerTest.php
@@ -0,0 +1,80 @@
+<?php
+
+namespace Drupal\Tests\Core\Messenger;
+
+use Drupal\Core\Messenger\StaticMessenger;
+use Drupal\Core\PageCache\ResponsePolicy\KillSwitch;
+use Drupal\Tests\RandomGeneratorTrait;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Messenger\StaticMessenger
+ * @group messenger
+ */
+class StaticMessengerTest extends \PHPUnit_Framework_TestCase {
+
+  use RandomGeneratorTrait;
+
+  /**
+   * The messenger under test.
+   *
+   * @var \Drupal\Core\Messenger\StaticMessenger
+   */
+  protected $messenger;
+
+  /**
+   * The page caching kill switch.
+   *
+   * @var \Drupal\Core\PageCache\ResponsePolicy\KillSwitch|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $pageCacheKillSwitch;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    $this->pageCacheKillSwitch = $this->prophesize(KillSwitch::class);
+  }
+
+  /**
+   * @covers ::addMessage
+   * @covers ::getMessages
+   * @covers ::getMessagesByType
+   * @covers ::deleteMessages
+   * @covers ::deleteMessagesByType
+   */
+  public function testMessenger() {
+    $message_a = $this->randomMachineName();
+    $type_a = $this->randomMachineName();
+    $message_b = $this->randomMachineName();
+    $type_b = $this->randomMachineName();
+
+    $this->pageCacheKillSwitch->trigger()->shouldBeCalled();
+
+    $this->messenger = new StaticMessenger($this->pageCacheKillSwitch->reveal());
+
+    // Test that if there are no messages, the default is an empty array.
+    $this->assertEquals($this->messenger->getMessages(), []);
+
+    // Test that adding a message returns the messenger and that the message can
+    // be retrieved.
+    $this->assertSame($this->messenger->addMessage($message_a, $type_a), $this->messenger);
+    $this->messenger->addMessage($message_a, $type_a);
+    $this->messenger->addMessage($message_a, $type_a, TRUE);
+    $this->messenger->addMessage($message_b, $type_b, TRUE);
+    $this->assertEquals([
+      $type_a => [$message_a, $message_a],
+      $type_b => [$message_b],
+    ], $this->messenger->getMessages());
+
+    // Test deleting messages of a certain type.
+    $this->assertEquals($this->messenger->deleteMessagesByType($type_a), $this->messenger);
+    $this->assertEquals([
+      $type_b => [$message_b],
+    ], $this->messenger->getMessages());
+
+    // Test deleting all messages.
+    $this->assertEquals($this->messenger->deleteMessages(), $this->messenger);
+    $this->assertEquals([], $this->messenger->getMessages());
+  }
+
+}