reverted:
--- b/core/lib/Drupal/Core/Entity/Field/FieldItemList.php
+++ a/core/lib/Drupal/Core/Entity/Field/FieldItemList.php
@@ -7,9 +7,7 @@
 
 namespace Drupal\Core\Entity\Field;
 
-use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Entity\Field\FieldItemListInterface;
-use Drupal\Core\Entity\Field\FieldDefinition;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\TypedData\TypedDataInterface;
 use Drupal\Core\TypedData\ItemList;
@@ -82,7 +80,7 @@
    * {@inheritdoc}
    */
   public function getFieldDefinition() {
+    return new FieldDefinition($this->definition);
-    return new FieldDefinition(NestedArray::mergeDeep($this->getPluginDefinition(), $this->definition));
   }
 
   /**
reverted:
--- b/core/lib/Drupal/Core/Entity/Plugin/DataType/Deriver/FieldItemDeriver.php
+++ a/core/lib/Drupal/Core/Entity/Plugin/DataType/Deriver/FieldItemDeriver.php
@@ -77,16 +77,6 @@
    */
   public function getDerivativeDefinitions(array $base_plugin_definition) {
     foreach ($this->fieldTypePluginManager->getDefinitions() as $plugin_id => $definition) {
-      // Provide easy access to the field type without requiring consuming code
-      // to parse it from the full data type.
-      $definition['field_type'] = $plugin_id;
-
-      // The distinction between 'settings' and 'instance_settings' is only
-      // meaningful at the field type plugin level. At the Typed data API level,
-      // merge them.
-      $definition['settings'] = $definition['instance_settings'] + $definition['settings'];
-      unset($definition['instance_settings']);
-
       $this->derivatives[$plugin_id] = $definition;
     }
     return $this->derivatives;
diff -u b/core/modules/edit/lib/Drupal/edit/EditController.php b/core/modules/edit/lib/Drupal/edit/EditController.php
--- b/core/modules/edit/lib/Drupal/edit/EditController.php
+++ b/core/modules/edit/lib/Drupal/edit/EditController.php
@@ -134,15 +134,12 @@
       }
 
       // Validate the field name and language.
-      if (!$field_name) {
+      if (!$field_name || || !$entity->getPropertyDefinition($field_name)) {
         throw new NotFoundHttpException();
       }
       if (!$langcode || (field_valid_language($langcode) !== $langcode)) {
         throw new NotFoundHttpException();
       }
-      if (!($field_definition = $entity->getTranslation($langcode)->get($field_name)->getFieldDefinition())) {
-        throw new NotFoundHttpException();
-      }
 
       // If the entity information for this field is requested, include it.
       $entity_id = $entity->entityType() . '/' . $entity_id;
@@ -150,6 +147,7 @@
         $metadata[$entity_id] = $this->metadataGenerator->generateEntity($entity, $langcode);
       }
 
+      $field_definition = $entity->get($field_name)->getFieldDefinition();
       $metadata[$field] = $this->metadataGenerator->generateField($entity, $field_definition, $langcode, $view_mode);
     }
 
diff -u b/core/modules/node/lib/Drupal/node/Entity/Node.php b/core/modules/node/lib/Drupal/node/Entity/Node.php
--- b/core/modules/node/lib/Drupal/node/Entity/Node.php
+++ b/core/modules/node/lib/Drupal/node/Entity/Node.php
@@ -363,6 +363,8 @@
       'required' => TRUE,
       'settings' => array(
         'default_value' => '',
+        'max_length' => 255,
+        'text_processing' => 0,
       ),
       'translatable' => TRUE,
     );