diff -u b/core/tests/Drupal/KernelTests/Core/Messenger/LegacyMessengerTest.php b/core/tests/Drupal/KernelTests/Core/Messenger/LegacyMessengerTest.php
--- b/core/tests/Drupal/KernelTests/Core/Messenger/LegacyMessengerTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Messenger/LegacyMessengerTest.php
@@ -17,8 +17,10 @@
    * Retrieves the Messenger service from LegacyMessenger.
    *
    * @param \Drupal\Core\Messenger\LegacyMessenger $legacy_messenger
+   *   The legacy messenger.
    *
    * @return \Drupal\Core\Messenger\MessengerInterface|null
+   *   A messenger implementation.
    */
   protected function getMessengerService(LegacyMessenger $legacy_messenger) {
     $method = new \ReflectionMethod($legacy_messenger, 'getMessengerService');
@@ -81,7 +83,12 @@
     $this->assertContains('Cheetah', $messages[MessengerInterface::TYPE_WARNING]);
   }
 
-
+  /**
+   * @covers ::all
+   * @covers ::addStatus
+   * @covers ::addWarning
+   * @covers ::addError
+   */
   public function testRepeatedMessages() {
     // Add repeated messages of different types.
     $messenger = \Drupal::messenger();
reverted:
--- b/core/tests/Drupal/KernelTests/Core/Messenger/MessengerKernelTest.php
+++ /dev/null
@@ -1,155 +0,0 @@
-<?php
-
-namespace Drupal\KernelTests\Core\Messenger;
-
-use Drupal\Core\Messenger\MessengerInterface;
-use Drupal\KernelTests\KernelTestBase;
-use Drupal\Core\Render\Markup;
-
-/**
- * @group Messenger
- * @coversDefaultClass \Drupal\Core\Messenger\Messenger
- */
-class MessengerKernelTest extends KernelTestBase {
-
-  /**
-   * The messenger under test.
-   *
-   * @var \Drupal\Core\Messenger\MessengerInterface
-   */
-  protected $messenger;
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function setUp() {
-    parent::setUp();
-    $this->messenger = \Drupal::service('messenger');
-  }
-
-  /**
-   * @covers ::addStatus
-   * @covers ::deleteByType
-   * @covers ::messagesByType
-   */
-  public function testRemoveSingleMessage() {
-
-    // Set two messages.
-    $this->messenger->addStatus('First message (removed).');
-    $this->messenger->addStatus(t('Second message with <em>markup!</em> (not removed).'));
-    $messages = $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS);
-    // Remove the first.
-    unset($messages[0]);
-
-    // Re-add the second.
-    foreach ($messages as $message) {
-      $this->messenger->addStatus($message);
-    }
-
-    // Check we only have the second one.
-    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
-    $this->assertContains('Second message with <em>markup!</em> (not removed).', $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
-
-  }
-
-  /**
-   * Tests we don't add duplicates.
-   *
-   * @covers ::all
-   * @covers ::addStatus
-   * @covers ::addWarning
-   * @covers ::addError
-   * @covers ::deleteByType
-   * @covers ::deleteAll
-   */
-  public function testAddNoDuplicates() {
-
-    $this->messenger->addStatus('Non Duplicated status message');
-    $this->messenger->addStatus('Non Duplicated status message');
-
-    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
-
-    $this->messenger->addWarning('Non Duplicated warning message');
-    $this->messenger->addWarning('Non Duplicated warning message');
-
-    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_WARNING));
-
-    $this->messenger->addError('Non Duplicated error message');
-    $this->messenger->addError('Non Duplicated error message');
-
-    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_ERROR));
-
-    // Check getting all messages.
-    $messages = $this->messenger->all();
-    $this->assertCount(3, $messages);
-    $this->assertArrayHasKey(MessengerInterface::TYPE_STATUS, $messages);
-    $this->assertArrayHasKey(MessengerInterface::TYPE_WARNING, $messages);
-    $this->assertArrayHasKey(MessengerInterface::TYPE_ERROR, $messages);
-
-    // Check deletion.
-    $this->messenger->deleteAll();
-    $this->assertCount(0, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
-    $this->assertCount(0, $this->messenger->messagesByType(MessengerInterface::TYPE_WARNING));
-    $this->assertCount(0, $this->messenger->messagesByType(MessengerInterface::TYPE_ERROR));
-
-  }
-
-  /**
-   * Tests we do add duplicates with repeat flag.
-   *
-   * @covers ::addStatus
-   * @covers ::addWarning
-   * @covers ::addError
-   * @covers ::deleteByType
-   */
-  public function testAddWithDuplicates() {
-
-    $this->messenger->addStatus('Duplicated status message', TRUE);
-    $this->messenger->addStatus('Duplicated status message', TRUE);
-
-    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
-
-    $this->messenger->addWarning('Duplicated warning message', TRUE);
-    $this->messenger->addWarning('Duplicated warning message', TRUE);
-
-    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_WARNING));
-
-    $this->messenger->addError('Duplicated error message', TRUE);
-    $this->messenger->addError('Duplicated error message', TRUE);
-
-    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_ERROR));
-
-  }
-
-  /**
-   * Test adding markup.
-   *
-   * @covers ::addStatus
-   * @covers ::deleteByType
-   * @covers ::messagesByType
-   */
-  public function testAddMarkup() {
-
-    // Add a Markup message.
-    $this->messenger->addStatus(Markup::create('Markup with <em>markup!</em>'));
-    // Test duplicate Markup messages.
-    $this->messenger->addStatus(Markup::create('Markup with <em>markup!</em>'));
-
-    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
-
-    // Ensure that multiple Markup messages work.
-    $this->messenger->addStatus(Markup::create('Markup2 with <em>markup!</em>'));
-
-    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
-
-    // Test mixing of types.
-    $this->messenger->addStatus(Markup::create('Non duplicate Markup / string.'));
-    $this->messenger->addStatus('Non duplicate Markup / string.');
-    $this->messenger->addStatus(Markup::create('Duplicate Markup / string.'), TRUE);
-    $this->messenger->addStatus('Duplicate Markup / string.', TRUE);
-
-    $this->assertCount(3, $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
-
-  }
-
-}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/tests/Drupal/KernelTests/Core/Messenger/MessengerTest.php
@@ -0,0 +1,165 @@
+<?php
+
+namespace Drupal\KernelTests\Core\Messenger;
+
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\Core\Render\Markup;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * @group Messenger
+ * @coversDefaultClass \Drupal\Core\Messenger\Messenger
+ */
+class MessengerTest extends KernelTestBase {
+
+  /**
+   * The messenger under test.
+   *
+   * @var \Drupal\Core\Messenger\MessengerInterface
+   */
+  protected $messenger;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->messenger = \Drupal::service('messenger');
+  }
+
+  /**
+   * @covers ::addStatus
+   * @covers ::deleteByType
+   * @covers ::messagesByType
+   */
+  public function testRemoveSingleMessage() {
+
+    // Set two messages.
+    $this->messenger->addStatus('First message (removed).');
+    $this->messenger->addStatus(t('Second message with <em>markup!</em> (not removed).'));
+    $messages = $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS);
+    // Remove the first.
+    unset($messages[0]);
+
+    // Re-add the second.
+    foreach ($messages as $message) {
+      $this->messenger->addStatus($message);
+    }
+
+    // Check we only have the second one.
+    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
+    $this->assertContains('Second message with <em>markup!</em> (not removed).', $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
+
+  }
+
+  /**
+   * Tests we don't add duplicates.
+   *
+   * @covers ::all
+   * @covers ::addStatus
+   * @covers ::addWarning
+   * @covers ::addError
+   * @covers ::deleteByType
+   * @covers ::deleteAll
+   */
+  public function testAddNoDuplicates() {
+
+    $this->messenger->addStatus('Non Duplicated status message');
+    $this->messenger->addStatus('Non Duplicated status message');
+
+    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
+
+    $this->messenger->addWarning('Non Duplicated warning message');
+    $this->messenger->addWarning('Non Duplicated warning message');
+
+    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_WARNING));
+
+    $this->messenger->addError('Non Duplicated error message');
+    $this->messenger->addError('Non Duplicated error message');
+
+    $messages = $this->messenger->messagesByType(MessengerInterface::TYPE_ERROR);
+    $this->assertCount(1, $messages);
+
+    // Check getting all messages.
+    $messages = $this->messenger->all();
+    $this->assertCount(3, $messages);
+    $this->assertArrayHasKey(MessengerInterface::TYPE_STATUS, $messages);
+    $this->assertArrayHasKey(MessengerInterface::TYPE_WARNING, $messages);
+    $this->assertArrayHasKey(MessengerInterface::TYPE_ERROR, $messages);
+
+    // Check deletion.
+    $this->messenger->deleteAll();
+    $this->assertCount(0, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
+    $this->assertCount(0, $this->messenger->messagesByType(MessengerInterface::TYPE_WARNING));
+    $this->assertCount(0, $this->messenger->messagesByType(MessengerInterface::TYPE_ERROR));
+
+  }
+
+  /**
+   * Tests we do add duplicates with repeat flag.
+   *
+   * @covers ::addStatus
+   * @covers ::addWarning
+   * @covers ::addError
+   * @covers ::deleteByType
+   */
+  public function testAddWithDuplicates() {
+
+    $this->messenger->addStatus('Duplicated status message', TRUE);
+    $this->messenger->addStatus('Duplicated status message', TRUE);
+
+    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
+
+    $this->messenger->addWarning('Duplicated warning message', TRUE);
+    $this->messenger->addWarning('Duplicated warning message', TRUE);
+
+    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_WARNING));
+
+    $this->messenger->addError('Duplicated error message', TRUE);
+    $this->messenger->addError('Duplicated error message', TRUE);
+
+    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_ERROR));
+
+  }
+
+  /**
+   * Test adding markup.
+   *
+   * @covers ::addStatus
+   * @covers ::deleteByType
+   * @covers ::messagesByType
+   */
+  public function testAddMarkup() {
+
+    // Add a Markup message.
+    $this->messenger->addStatus(Markup::create('Markup with <em>markup!</em>'));
+    // Test duplicate Markup messages.
+    $this->messenger->addStatus(Markup::create('Markup with <em>markup!</em>'));
+
+    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
+
+    // Ensure that multiple Markup messages work.
+    $this->messenger->addStatus(Markup::create('Markup2 with <em>markup!</em>'));
+
+    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
+
+    // Test mixing of types.
+    $this->messenger->addStatus(Markup::create('Non duplicate Markup / string.'));
+    $this->messenger->addStatus('Non duplicate Markup / string.');
+    $this->messenger->addStatus(Markup::create('Duplicate Markup / string.'), TRUE);
+    $this->messenger->addStatus('Duplicate Markup / string.', TRUE);
+
+    $this->assertCount(3, $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
+
+    $this->messenger->deleteAll();
+
+    // Check translatable string is converted to Markup.
+    $this->messenger->addStatus(new TranslatableMarkup('Translatable message'));
+    $messages = $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS);
+
+    $this->assertInstanceOf(Markup::class, $messages[0]);
+
+  }
+
+}