diff --git a/core/tests/Drupal/KernelTests/Core/Test/AssertMailTraitTest.php b/core/tests/Drupal/KernelTests/Core/Test/AssertMailTraitTest.php
index e368ead..7af7718 100644
--- a/core/tests/Drupal/KernelTests/Core/Test/AssertMailTraitTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Test/AssertMailTraitTest.php
@@ -24,6 +24,9 @@ class AssertMailTraitTest extends KernelTestBase {
    * Tests that the maintenance theme initializes the theme and its base themes.
    */
   public function testAssertMailTrait() {
+    /* @var \Drupal\Core\Mail\MailManagerInterface $mail_service */
+    $mail_service = \Drupal::service('plugin.manager.mail');
+
     // Create an email.
     $subject = $this->randomString(64);
     $body = $this->randomString(128);
@@ -35,12 +38,13 @@ public function testAssertMailTrait() {
       'body' => $body,
     ];
 
-    // Before we send the email, drupalGetMails should return an empty array.
+    // Before we send the email, \Drupal\Core\Test\AssertMailTrait::getMails()
+    // should return an empty array.
     $captured_emails = $this->getMails();
     $this->assertCount(0, $captured_emails, 'The captured emails queue is empty.');
 
     // Send the email.
-    \Drupal::service('plugin.manager.mail')->getInstance(['module' => 'simpletest', 'key' => 'drupal_mail_test'])->mail($message);
+    $mail_service->getInstance(['module' => 'simpletest', 'key' => 'drupal_mail_test'])->mail($message);
 
     // Ensure that there is one email in the captured emails array.
     $captured_emails = $this->getMails();
@@ -49,7 +53,7 @@ public function testAssertMailTrait() {
     // Assert that the email was sent by iterating over the message properties
     // and ensuring that they are captured intact.
     foreach ($message as $field => $value) {
-      $this->assertMail($field, $value, sprintf('The email was sent and the value for property %s is intact.', $field));
+      $this->assertMail($field, $value, "The email was sent and the value for property $field is intact.");
     }
 
     // Send additional emails so more than one email is captured.
@@ -61,14 +65,15 @@ public function testAssertMailTrait() {
         'to' => $this->randomMachineName(32) . '@example.com',
         'body' => $this->randomString(512),
       ];
-      \Drupal::service('plugin.manager.mail')->getInstance(['module' => 'drupal_mail_test', 'key' => $index])->mail($message);
+      $mail_service->getInstance(['module' => 'drupal_mail_test', 'key' => $index])->mail($message);
     }
 
     // There should now be 6 emails captured.
     $captured_emails = $this->getMails();
     $this->assertCount(6, $captured_emails, 'All emails were captured.');
 
-    // Test different ways of getting filtered emails via drupalGetMails().
+    // Test different ways of getting filtered emails via
+    // \Drupal\Core\Test\AssertMailTrait::getMails().
     $captured_emails = $this->getMails(['id' => 'drupal_mail_test']);
     $this->assertCount(1, $captured_emails, 'Only one email is returned when filtering by id.');
     $captured_emails = $this->getMails(['id' => 'drupal_mail_test', 'subject' => $subject]);
@@ -80,10 +85,10 @@ public function testAssertMailTrait() {
     ]);
     $this->assertCount(0, $captured_emails, 'No emails are returned when querying with an unused from address.');
 
-    // Send the last email again, so we can confirm that the
-    // drupalGetMails-filter correctly returns all emails with a given
-    // property/value.
-    \Drupal::service('plugin.manager.mail')->getInstance(['module' => 'drupal_mail_test', 'key' => $index])->mail($message);
+    // Send the last email again, so we can confirm that
+    // \Drupal\Core\Test\AssertMailTrait::getMails() filters correctly returns
+    // all emails with a given property/value.
+    $mail_service->getInstance(['module' => 'drupal_mail_test', 'key' => $index])->mail($message);
     $captured_emails = $this->getMails(['id' => 'drupal_mail_test_4']);
     $this->assertCount(2, $captured_emails, 'All emails with the same id are returned when filtering by id.');
   }