diff --git a/core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php b/core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php
index 194b3f5..7905045 100644
--- a/core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php
+++ b/core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php
@@ -17,8 +17,6 @@
  */
 abstract class ListItemBase extends FieldItemBase implements AllowedValuesInterface {
 
-  use ListItemAllowedValuesTrait;
-
   /**
    * {@inheritdoc}
    */
@@ -248,4 +246,53 @@ protected function allowedValuesString($structured_values) {
     return implode("\n", $lines);
   }
 
+  /**
+   * Simplify allowed values to a key-value array from the structured array.
+   *
+   * @param array $structured_values
+   *   Array of items with a 'value' and 'label' key each for the allowed
+   *   values.
+   *
+   * @return array
+   *   Allowed values were the array key is the 'value' value, the value is
+   *   the 'label' value.
+   */
+  public static function simplifyAllowedValues(array $structured_values) {
+    $values = array();
+    foreach ($structured_values as $item) {
+      if (is_array($item['label'])) {
+        // Nested elements are embedded in the label.
+        // See structureAllowedValues().
+        $item['label'] = static::simplifyAllowedValues($item['label']);
+      }
+      $values[$item['value']] = $item['label'];
+    }
+    return $values;
+  }
+
+  /**
+   * Creates a structured array of allowed values from a key-value array.
+   *
+   * @param array $values
+   *   Allowed values were the array key is the 'value' value, the value is
+   *   the 'label' value.
+   *
+   * @return array
+   *   Array of items with a 'value' and 'label' key each for the allowed
+   *   values.
+   */
+  public static function structureAllowedValues(array $values) {
+    $structured_values = array();
+    foreach ($values as $value => $label) {
+      if (is_array($label)) {
+        $label = static::structureAllowedValues($label);
+      }
+      $structured_values[] = array(
+        'value' => $value,
+        'label' => $label,
+      );
+    }
+    return $structured_values;
+  }
+
 }