reverted:
--- b/core/tests/Drupal/KernelTests/Core/Database/MergeTest.php
+++ a/core/tests/Drupal/KernelTests/Core/Database/MergeTest.php
@@ -18,8 +18,7 @@
   public function testMergeInsert() {
     $num_records_before = db_query('SELECT COUNT(*) FROM {test_people}')->fetchField();
 
+    $result = db_merge('test_people')
-    // @todo Inject database service https://www.drupal.org/node/2947769
-    $result = \Drupal::database()->merge('test_people')
       ->key('job', 'Presenter')
       ->fields([
         'age' => 31,
@@ -44,7 +43,7 @@
   public function testMergeUpdate() {
     $num_records_before = db_query('SELECT COUNT(*) FROM {test_people}')->fetchField();
 
+    $result = db_merge('test_people')
-    $result = \Drupal::database()->merge('test_people')
       ->key('job', 'Speaker')
       ->fields([
         'age' => 31,
@@ -72,7 +71,7 @@
   public function testMergeUpdateExcept() {
     $num_records_before = db_query('SELECT COUNT(*) FROM {test_people}')->fetchField();
 
+    db_merge('test_people')
-    \Drupal::database()->merge('test_people')
       ->key('job', 'Speaker')
       ->insertFields(['age' => 31])
       ->updateFields(['name' => 'Tiffany'])
@@ -93,7 +92,7 @@
   public function testMergeUpdateExplicit() {
     $num_records_before = db_query('SELECT COUNT(*) FROM {test_people}')->fetchField();
 
+    db_merge('test_people')
-    \Drupal::database()->merge('test_people')
       ->key('job', 'Speaker')
       ->insertFields([
         'age' => 31,
@@ -126,7 +125,7 @@
     // Note that we are also double-setting age here, once as a literal and
     // once as an expression. This test will only pass if the expression wins,
     // which is what is supposed to happen.
+    db_merge('test_people')
-    \Drupal::database()->merge('test_people')
       ->key('job', 'Speaker')
       ->fields(['name' => 'Tiffany'])
       ->insertFields(['age' => 31])
@@ -148,7 +147,7 @@
   public function testMergeInsertWithoutUpdate() {
     $num_records_before = db_query('SELECT COUNT(*) FROM {test_people}')->fetchField();
 
+    db_merge('test_people')
-    \Drupal::database()->merge('test_people')
       ->key('job', 'Presenter')
       ->execute();
 
@@ -167,7 +166,7 @@
   public function testMergeUpdateWithoutUpdate() {
     $num_records_before = db_query('SELECT COUNT(*) FROM {test_people}')->fetchField();
 
+    db_merge('test_people')
-    \Drupal::database()->merge('test_people')
       ->key('job', 'Speaker')
       ->execute();
 
@@ -179,7 +178,7 @@
     $this->assertEqual($person->age, 30, 'Age skipped correctly.');
     $this->assertEqual($person->job, 'Speaker', 'Job skipped correctly.');
 
+    db_merge('test_people')
-    \Drupal::database()->merge('test_people')
       ->key('job', 'Speaker')
       ->insertFields(['age' => 31])
       ->execute();
@@ -202,7 +201,7 @@
       // Normally it would throw an exception but we are suppressing it with
       // the throw_exception option.
       $options['throw_exception'] = FALSE;
+      db_merge('test_people', $options)
-      \Drupal::database()->merge('test_people', $options)
         ->fields([
           'age' => 31,
           'name' => 'Tiffany',
@@ -217,7 +216,7 @@
 
     try {
       // This query will fail because there is no key field specified.
+      db_merge('test_people')
-      \Drupal::database()->merge('test_people')
         ->fields([
           'age' => 31,
           'name' => 'Tiffany',