Clean up entity_reference default values once users 0 and 1 provides UUID

Thanks for following up on this, @plopesc.

The extra dependency is a bit unfortunate IMO.

But, in the same spirit than #2156337: merge ConfigEntityReferenceItemBase up into EntityReferenceItem, and fix inconsistencies, I think the methods could be moved to an EntityReferenceItem*List* class at the level of the \Core EntityRef field type (Core\Field\Plugin\Field\FieldType\EntityReferenceItem), that all other "reference" field types would use too.

(as a side note, I bumped #2073661: Add a EntityReferenceField::referencedEntities() method, which also has a case for introducing an EntityReferenceItemList class)

5: clean_user_uuid_exception-2162005-5.patch queued for re-testing.

I think this can be unpostponed.

Awesome !
+ removal of two specialized list classes : awesome++

Committed f2015b7 and pushed to 8.x. Thanks!

+++ b/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php
@@ -46,4 +49,69 @@ public function referencedEntities() {
+
+        foreach ($entities as $id => $entity) {
+          $entity_uuids[$entity->uuid()] = $id;
+        }
+        foreach ($uuids as $delta => $uuid) {
+          if ($entity_uuids[$uuid]) {
+            $default_value[$delta]['target_id'] = $entity_uuids[$uuid];
+            unset($default_value[$delta]['target_uuid']);
+          }

$entity_uuids is not defined and is not doing an isset() check, so if targets actually don't exist, this is throwing php notices.

Good to go when green.

Committed a5e7c59 and pushed to 8.0.x. Thanks!