Convert form_options_flatten() to a method on FormBuilder

+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -1647,6 +1654,45 @@ public function setValue($element, $value, &$form_state) {
+  /**
+   * Iterates over an array and returns a flat array with duplicate keys removed.
+   *
+   * This function also handles cases where objects are passed as array values.
+   */
+  protected function doFlattenOptions($array) {

Should we also document the @param @return here?

+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -1656,29 +1656,23 @@ public function setValue($element, $value, &$form_state) {
+   * Iterates over an array returning a flat array with duplicate keys removed.

Well ... we don't really return anything.

Thank you!

+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -125,6 +125,13 @@ class FormBuilder implements FormBuilderInterface {
+  protected $flattenedOptions = array();

@@ -1649,6 +1656,37 @@ public function setValue($element, $value, &$form_state) {
+    $this->flattenedOptions = array();
...
+  protected function doFlattenOptions(array $array) {
...
+        $this->flattenedOptions[$key] = 1;

Just putting it out there, instead of essentially storing state on the form builder server, could we pass the flattened options as a second parameter? this could be passed recursively etc... no problem.

Let me know if I'm talking shit.

listed which number followup this was

6: form-2120841-6.patch queued for re-testing.

#2138803: Overhaul field items' flattenOption is postponed on this issue.

Committed and pushed to 8.x. Thanks!