Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

core/modules/field/src/FieldStorageConfigInterface.php

@@ -96,10 +96,32 @@ interface FieldStorageConfigInterface extends ConfigEntityInterface, FieldStorag
   public function setSetting($setting_name, $value);
 
   /**
-   * Sets field settings (overwrites existing settings).
+   * Sets field storage settings.
+   *
+   * Note that the method does not unset existing settings not specified in the
+   * incoming $settings array.
+   *
+   * For example:
+   * @code
+   *   // Given these are the default settings.
+   *   $storage_definition->getSettings() === [
+   *     'fruit' => 'apple',
+   *     'season' => 'summer',
+   *   ];
+   *   // Change only the 'fruit' setting.
+   *   $storage_definition->setSettings(['fruit' => 'banana']);
+   *   // The 'season' setting persists unchanged.
+   *   $storage_definition->getSettings() === [
+   *     'fruit' => 'banana',
+   *     'season' => 'summer',
+   *   ];
+   * @endcode
+   *
+   * For clarity, it is preferred to use setSetting() if not all available
+   * settings are supplied.
    *
    * @param array $settings
-   *   The array of field settings.
+   *   The array of storage settings.
    *
    * @return $this
    */