reverted:
--- b/src/TermMergeEvent.php	(revision ee78b9915256b871ac1e37a2a3f11baedc43e614)
+++ a/src/TermMergeEvent.php	(revision ee78b9915256b871ac1e37a2a3f11baedc43e614)
@@ -1,84 +0,0 @@
-<?php
-
-namespace Drupal\term_merge;
-
-use Drupal\taxonomy\Entity\Term;
-use Symfony\Component\EventDispatcher\Event;
-
-/**
- * Event used for Merges.
- *
- * Class TermMergeEvent.
- *
- * @package Drupal\term_merge
- */
-class TermMergeEvent extends Event {
-
-  /**
-   * Array of terms. These terms are getting merged into $targetTerm.
-   *
-   * @var array
-   */
-  protected $termsToMerge;
-
-  /**
-   * Target Term. All $termsToMerge are getting merged into this.
-   *
-   * @var \Drupal\taxonomy\Entity\Term
-   */
-  protected $targetTerm;
-
-  /**
-   * Constructor.
-   *
-   * @param array $termsToMerge
-   *   Terms to merge.
-   * @param \Drupal\taxonomy\Entity\Term $targetTerm
-   *   Target Term.
-   */
-  public function __construct(array $termsToMerge, Term $targetTerm) {
-    $this->termsToMerge = $termsToMerge;
-    $this->targetTerm = $targetTerm;
-  }
-
-  /**
-   * Get all terms to merge.
-   *
-   * @return array
-   *   An array of terms to merge.
-   */
-  public function getTermsToMerge() {
-    return $this->termsToMerge;
-  }
-
-  /**
-   * Get the single target term.
-   *
-   * @return \Drupal\taxonomy\Entity\Term
-   *   Single target term object.
-   */
-  public function getTargetTerm() {
-    return $this->targetTerm;
-  }
-
-  /**
-   * Set/Update terms to merge.
-   *
-   * @param array $termsToMerge
-   *   An array of terms to merge.
-   */
-  public function setTermsToMerge(array $termsToMerge) {
-    $this->termsToMerge = $termsToMerge;
-  }
-
-  /**
-   * Set/Update the target term.
-   *
-   * @param \Drupal\taxonomy\Entity\Term $targetTerm
-   *   Single target term object.
-   */
-  public function setTargetTerm(Term $targetTerm) {
-    $this->targetTerm = $targetTerm;
-  }
-
-}
only in patch2:
unchanged:
--- /dev/null
+++ b/src/TermMergeEvent.php
@@ -0,0 +1,64 @@
+<?php
+
+namespace Drupal\term_merge;
+
+use Drupal\taxonomy\Entity\Term;
+use Symfony\Component\EventDispatcher\Event;
+
+/**
+ * Event used for Merges.
+ *
+ * Class TermMergeEvent.
+ *
+ * @package Drupal\term_merge
+ */
+class TermMergeEvent extends Event {
+
+  /**
+   * Array of terms. These terms are getting merged into $targetTerm.
+   *
+   * @var array
+   */
+  protected $termsToMerge;
+
+  /**
+   * Target Term. All $termsToMerge are getting merged into this.
+   *
+   * @var \Drupal\taxonomy\Entity\Term
+   */
+  protected $targetTerm;
+
+  /**
+   * Constructor.
+   *
+   * @param array $termsToMerge
+   *   Terms to merge.
+   * @param \Drupal\taxonomy\Entity\Term $targetTerm
+   *   Target Term.
+   */
+  public function __construct(array $termsToMerge, Term $targetTerm) {
+    $this->termsToMerge = $termsToMerge;
+    $this->targetTerm = $targetTerm;
+  }
+
+  /**
+   * Get all terms to merge.
+   *
+   * @return array
+   *   An array of terms to merge.
+   */
+  public function getTermsToMerge() {
+    return $this->termsToMerge;
+  }
+
+  /**
+   * Get the single target term.
+   *
+   * @return \Drupal\taxonomy\Entity\Term
+   *   Single target term object.
+   */
+  public function getTargetTerm() {
+    return $this->targetTerm;
+  }
+
+}
only in patch2:
unchanged:
--- a/tests/src/Functional/IntegrationTest.php
+++ b/tests/src/Functional/IntegrationTest.php
@@ -20,7 +20,7 @@ class IntegrationTest extends BrowserTestBase {
   /**
    * {@inheritdoc}
    */
-  public static $modules = ['node', 'taxonomy', 'term_merge'];
+  public static $modules = ['node', 'taxonomy', 'term_merge', 'term_merge_test_events'];
 
   /**
    * The content type.
@@ -112,6 +112,10 @@ class IntegrationTest extends BrowserTestBase {
     $this->assertSession()->statusCodeEquals(200);
     $this->assertSession()
       ->pageTextContains("Successfully merged 1 terms into {$b->label()}");
+    // The message below is printed to the screen by an event subscriber
+    // located in the term_merge_test_events test module.
+    $this->assertSession()
+      ->pageTextContains('The TermMergeEvent was triggered.');
   }
 
   /**
only in patch2:
unchanged:
--- /dev/null
+++ b/tests/term_merge_test_events/src/EventSubscriber/TermMergeEventSubscriber.php
@@ -0,0 +1,33 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\term_merge_test_events\EventSubscriber\TermMergeEventSubscriber.
+ */
+
+namespace Drupal\term_merge_test_events\EventSubscriber;
+
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Drupal\term_merge\TermMergeEvent;
+
+/**
+ * The TermMergeEventSubscriber event subscriber.
+ */
+class TermMergeEventSubscriber implements EventSubscriberInterface {
+
+  /**
+   * Code that is executed when the event is triggered.
+   */
+  public function onTermMerge(TermMergeEvent $event) {
+    \Drupal::messenger()->addMessage('The TermMergeEvent was triggered.');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    $events['term_merge.merge_action'][] = ['onTermMerge'];
+    return $events;
+  }
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/tests/term_merge_test_events/term_merge_test_events.info.yml
@@ -0,0 +1,8 @@
+name: 'Term merge test events'
+type: module
+description: 'Helps to test events triggered in the term_merge module by creating event subscribers.'
+package: Testing
+dependencies:
+  - term_merge:term_merge
+core: 8.x
+hidden: true
only in patch2:
unchanged:
--- /dev/null
+++ b/tests/term_merge_test_events/term_merge_test_events.services.yml
@@ -0,0 +1,5 @@
+services:
+  term_merge_event_subscriber:
+    class: '\Drupal\term_merge_test_events\EventSubscriber\TermMergeEventSubscriber'
+    tags:
+      - { name: event_subscriber }