diff -u b/core/modules/migrate/tests/src/Unit/MigrateSourceTest.php b/core/modules/migrate/tests/src/Unit/MigrateSourceTest.php
--- b/core/modules/migrate/tests/src/Unit/MigrateSourceTest.php
+++ b/core/modules/migrate/tests/src/Unit/MigrateSourceTest.php
@@ -313,7 +313,6 @@
     $module_handler->invokeAll('migrate_' . $migration->id() . '_prepare_row', [$row, $source, $migration])
       ->willReturn([TRUE, TRUE])
       ->shouldBeCalled();
-    $source->setModuleHandler($module_handler->reveal());
 
     $this->idMap->expects($this->once())
       ->method('saveIdMapping')
only in patch2:
unchanged:
--- a/core/modules/migrate_drupal/tests/src/Unit/source/d6/Drupal6SqlBaseTest.php
+++ b/core/modules/migrate_drupal/tests/src/Unit/source/d6/Drupal6SqlBaseTest.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\Tests\migrate_drupal\Unit\source\d6;
 
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Tests\migrate\Unit\MigrateTestCase;
 
 /**
@@ -68,11 +70,13 @@ class Drupal6SqlBaseTest extends MigrateTestCase {
    */
   protected function setUp() {
     $plugin = 'placeholder_id';
+    $module_handler = $this->getMock(ModuleHandlerInterface::class);
+    $cache = $this->getMock(CacheBackendInterface::class);
     /** @var \Drupal\Core\State\StateInterface $state */
     $state = $this->getMock('Drupal\Core\State\StateInterface');
     /** @var \Drupal\Core\Entity\EntityManagerInterface $entity_manager */
     $entity_manager = $this->getMock('Drupal\Core\Entity\EntityManagerInterface');
-    $this->base = new TestDrupal6SqlBase($this->migrationConfiguration, $plugin, array(), $this->getMigration(), $state, $entity_manager);
+    $this->base = new TestDrupal6SqlBase($this->migrationConfiguration, $plugin, array(), $this->getMigration(), $module_handler, $cache, $state, $entity_manager);
     $this->base->setDatabase($this->getDatabase($this->databaseContents));
   }
 
@@ -180,18 +184,6 @@ public function setDatabase(Connection $database) {
   }
 
   /**
-   * Tweaks Drupal6SqlBase to set a new module handler for tests.
-   *
-   * @param \Drupal\Core\Extension\ModuleHandlerInterface
-   *   The new module handler to use.
-   *
-   * @see \Drupal\Tests\migrate\Unit\MigrateSqlTestCase
-   */
-  public function setModuleHandler(ModuleHandlerInterface $module_handler) {
-    $this->moduleHandler = $module_handler;
-  }
-
-  /**
    * Wrapper method to test protected method moduleExists().
    */
   public function moduleExistsWrapper($module) {