diff -u b/core/modules/migrate/src/Tests/MigrateTestBase.php b/core/modules/migrate/src/Tests/MigrateTestBase.php
--- b/core/modules/migrate/src/Tests/MigrateTestBase.php
+++ b/core/modules/migrate/src/Tests/MigrateTestBase.php
@@ -74,21 +74,6 @@
   }
 
   /**
-   * Prepare the migration.
-   *
-   * @param \Drupal\migrate\Entity\MigrationInterface $migration
-   *   The migration object.
-   * @param array $files
-   *   An array of files.
-   */
-  protected function prepare(MigrationInterface $migration, array $files = array()) {
-    $this->loadDumps($files);
-    if ($this instanceof MigrateDumpAlterInterface) {
-      static::migrateDumpAlter($this);
-    }
-  }
-
-  /**
    * Load Drupal 6 database dumps to be used.
    *
    * @param array $files
diff -u b/core/modules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php b/core/modules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php
--- b/core/modules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php
+++ b/core/modules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php
@@ -9,6 +9,7 @@
 
 use Drupal\migrate\Entity\Migration;
 use Drupal\migrate\MigrateExecutable;
+use Drupal\migrate\Tests\MigrateDumpAlterInterface;
 use Drupal\migrate\Tests\MigrateTestBase;
 
 /**
@@ -62,6 +63,9 @@
     if (is_string($migration)) {
       $migration = Migration::load($migration);
     }
+    if ($this instanceof MigrateDumpAlterInterface) {
+      static::migrateDumpAlter($this);
+    }
     (new MigrateExecutable($migration, $this))->import();
   }