reverted:
--- b/core/includes/config.inc
+++ a/core/includes/config.inc
@@ -5,7 +5,6 @@
 use Drupal\Core\Config\FileStorage;
 use Drupal\Core\Config\NullStorage;
 use Drupal\Core\Config\StorageInterface;
-use Drupal\Core\Config\StorageException;
 
 /**
  * @file
diff -u b/core/lib/Drupal/Core/Config/StorageBase.php b/core/lib/Drupal/Core/Config/StorageBase.php
--- b/core/lib/Drupal/Core/Config/StorageBase.php
+++ b/core/lib/Drupal/Core/Config/StorageBase.php
@@ -25,7 +25,7 @@
   /**
    * Implements Drupal\Core\Config\StorageInterface::validateValue().
    */
-  public static function validateValue($value, $key, $name) {
+  public function validateValue($value, $key, $name) {
     if (!is_scalar($value)) {
       throw new StorageException(sprintf('Unsupported non-scalar value of type %s in key %s in config object %s', gettype($value), $key, $name));
     }
diff -u b/core/lib/Drupal/Core/Config/StorageInterface.php b/core/lib/Drupal/Core/Config/StorageInterface.php
--- b/core/lib/Drupal/Core/Config/StorageInterface.php
+++ b/core/lib/Drupal/Core/Config/StorageInterface.php
@@ -101,6 +101,8 @@
   public function validateData($name, array $data);
 
   /**
+   * Validates a configuration value.
+   *
    * Helper callback for array_walk_recursive().
    *
    * @param mixed $value
@@ -114,7 +116,7 @@
    *
    * @see StorageBase::validateData()
    */
-  public static function validateValue($value, $key, $name);
+  public function validateValue($value, $key, $name);
 
   /**
    * Gets configuration object names starting with a given prefix.