diff -u b/core/lib/Drupal/Core/Test/AssertMailTrait.php b/core/lib/Drupal/Core/Test/AssertMailTrait.php
--- b/core/lib/Drupal/Core/Test/AssertMailTrait.php
+++ b/core/lib/Drupal/Core/Test/AssertMailTrait.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * @file
  * Contains \Drupal\Core\Test\AssertMailTrait.
@@ -14,14 +15,14 @@
   /**
    * Gets an array containing all emails sent during this test case.
    *
-   * @param $filter
+   * @param array $filter
    *   An array containing key/value pairs used to filter the emails that are
    *   returned.
    *
    * @return array
    *   An array containing email messages captured during the current test.
    */
-  protected function getMails($filter = []) {
+  protected function getMails(array $filter = []) {
     $captured_emails = $this->container->get('state')->get('system.test_mail_collector', []);
     $filtered_emails = [];
 
@@ -42,23 +43,23 @@
    *
    * The field in $name must have the content described in $value.
    *
-   * @param $name
+   * @param string $name
    *   Name of field or message property to assert. Examples: subject, body,
    *   id, ...
-   * @param $value
+   * @param string $value
    *   Value of the field to assert.
-   * @param $message
+   * @param string $message
    *   (optional) A message to display with the assertion. Do not translate
    *   messages: use \Drupal\Component\Utility\SafeMarkup::format() to embed
    *   variables in the message text, not t(). If left blank, a default message
    *   will be displayed.
-   * @param $group
+   * @param string $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
    *   translate this string. Defaults to 'Email'; most tests do not override
    *   this default.
    *
-   * @return
+   * @return bool
    *   TRUE on pass, FALSE on fail.
    */
   protected function assertMail($name, $value = '', $message = '', $group = 'Email') {
@@ -70,24 +71,24 @@
   /**
    * Asserts that the most recently sent email message has the string in it.
    *
-   * @param $field_name
+   * @param string $field_name
    *   Name of field or message property to assert: subject, body, id, ...
-   * @param $string
+   * @param string $string
    *   String to search for.
-   * @param $email_depth
+   * @param int $email_depth
    *   Number of emails to search for string, starting with most recent.
-   * @param $message
+   * @param string $message
    *   (optional) A message to display with the assertion. Do not translate
    *   messages: use \Drupal\Component\Utility\SafeMarkup::format() to embed
    *   variables in the message text, not t(). If left blank, a default message
    *   will be displayed.
-   * @param $group
+   * @param string $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
    *   translate this string. Defaults to 'Other'; most tests do not override
    *   this default.
    *
-   * @return
+   * @return bool
    *   TRUE on pass, FALSE on fail.
    */
   protected function assertMailString($field_name, $string, $email_depth, $message = '', $group = 'Other') {
@@ -95,7 +96,7 @@
     $string_found = FALSE;
     // Cast MarkupInterface objects to string.
     $string = (string) $string;
-    for ($i = count($mails) -1; $i >= count($mails) - $email_depth && $i >= 0; $i--) {
+    for ($i = count($mails) - 1; $i >= count($mails) - $email_depth && $i >= 0; $i--) {
       $mail = $mails[$i];
       // Normalize whitespace, as we don't know what the mail system might have
       // done. Any run of whitespace becomes a single space.
@@ -115,22 +116,22 @@
   /**
    * Asserts that the most recently sent email message has the pattern in it.
    *
-   * @param $field_name
+   * @param string $field_name
    *   Name of field or message property to assert: subject, body, id, ...
-   * @param $regex
+   * @param string $regex
    *   Pattern to search for.
-   * @param $message
+   * @param string $message
    *   (optional) A message to display with the assertion. Do not translate
    *   messages: use \Drupal\Component\Utility\SafeMarkup::format() to embed
    *   variables in the message text, not t(). If left blank, a default message
    *   will be displayed.
-   * @param $group
+   * @param string $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
    *   translate this string. Defaults to 'Other'; most tests do not override
    *   this default.
    *
-   * @return
+   * @return bool
    *   TRUE on pass, FALSE on fail.
    */
   protected function assertMailPattern($field_name, $regex, $message = '', $group = 'Other') {
@@ -149,13 +150,12 @@
-   * @param $count
+   * @param int $count
    *   Optional number of emails to output.
    */
   protected function verboseEmail($count = 1) {
     $mails = $this->getMails();
-    for ($i = count($mails) -1; $i >= count($mails) - $count && $i >= 0; $i--) {
+    for ($i = count($mails) - 1; $i >= count($mails) - $count && $i >= 0; $i--) {
       $mail = $mails[$i];
       $this->verbose('Email:<pre>' . print_r($mail, TRUE) . '</pre>');
     }
   }
 
 }
-
only in patch2:
unchanged:
--- /dev/null
+++ b/core/tests/Drupal/KernelTests/Core/Test/AssertMailTraitTest.php
@@ -0,0 +1,91 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\KernelTests\Core\Test\AssertMailTraitTest.
+ */
+
+namespace Drupal\KernelTests\Core\Test;
+
+use Drupal\Core\Test\AssertMailTrait;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests \Drupal\Core\Test\AssertMailTrait works.
+ *
+ * @group Test
+ *
+ * @coversDefaultClass \Drupal\Core\Test\AssertMailTrait
+ */
+class AssertMailTraitTest extends KernelTestBase {
+  use AssertMailTrait;
+
+  /**
+   * Tests that the maintenance theme initializes the theme and its base themes.
+   */
+  public function testAssertMailTrait() {
+    // Create an email.
+    $subject = $this->randomString(64);
+    $body = $this->randomString(128);
+    $message = [
+      'id' => 'drupal_mail_test',
+      'headers' => ['Content-type' => 'text/html'],
+      'subject' => $subject,
+      'to' => 'foobar@example.com',
+      'body' => $body,
+    ];
+
+    // Before we send the email, drupalGetMails 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);
+
+    // Ensure that there is one email in the captured emails array.
+    $captured_emails = $this->getMails();
+    $this->assertEquals(count($captured_emails), 1, 'One email was captured.');
+
+    // 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.', $field));
+    }
+
+    // Send additional emails so more than one email is captured.
+    for ($index = 0; $index < 5; $index++) {
+      $message = [
+        'id' => 'drupal_mail_test_' . $index,
+        'headers' => ['Content-type' => 'text/html'],
+        'subject' => $this->randomString(64),
+        'to' => $this->randomMachineName(32) . '@example.com',
+        'body' => $this->randomString(512),
+      ];
+      \Drupal::service('plugin.manager.mail')->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().
+    $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]);
+    $this->assertCount(1, $captured_emails, 'Only one email is returned when filtering by id and subject.');
+    $captured_emails = $this->getMails([
+      'id' => 'drupal_mail_test',
+      'subject' => $subject,
+      'from' => 'this_was_not_used@example.com',
+    ]);
+    $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);
+    $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.');
+  }
+
+}