Issue #2181593 by klausi, chakrapani, andypost: Convert entity bundle base fields to entity reference.

core/modules/block/custom_block/lib/Drupal/custom_block/Entity/CustomBlock.php

@@ -188,9 +188,10 @@ class CustomBlock extends ContentEntityBase implements CustomBlockInterface {
       ->setLabel(t('Subject'))
       ->setDescription(t('The custom block name.'));
 
-    $fields['type'] = FieldDefinition::create('string')
+    $fields['type'] = FieldDefinition::create('entity_reference')
       ->setLabel(t('Block type'))
-      ->setDescription(t('The block type.'));
+      ->setDescription(t('The block type.'))
+      ->setSetting('target_type', 'custom_block_type');
 
     $fields['log'] = FieldDefinition::create('string')
       ->setLabel(t('Revision log message'))

core/modules/comment/lib/Drupal/comment/Entity/Comment.php

@@ -430,11 +430,10 @@ class Comment extends ContentEntityBase implements CommentInterface {
       ->setLabel(t('Entity type'))
       ->setDescription(t('The entity type to which this comment is attached.'));
 
-    // @todo Convert to aa entity_reference field in
-    // https://drupal.org/node/2149859.
-    $fields['field_id'] = FieldDefinition::create('string')
+    $fields['field_id'] = FieldDefinition::create('entity_reference')
       ->setLabel(t('Field ID'))
-      ->setDescription(t('The comment field id.'));
+      ->setDescription(t('The comment field id.'))
+      ->setSetting('target_type', 'field_entity');
 
     $fields['field_name'] = FieldDefinition::create('string')
       ->setLabel(t('Comment field name'))

core/modules/taxonomy/lib/Drupal/taxonomy/Entity/Term.php

@@ -208,11 +208,10 @@ class Term extends ContentEntityBase implements TermInterface {
       ->setDescription(t('The term UUID.'))
       ->setReadOnly(TRUE);
 
-    // @todo Convert this to an entity_reference field, see
-    //   https://drupal.org/node/2181593
-    $fields['vid'] = FieldDefinition::create('string')
-      ->setLabel(t('Vocabulary ID'))
-      ->setDescription(t('The ID of the vocabulary to which the term is assigned.'));
+    $fields['vid'] = FieldDefinition::create('entity_reference')
+      ->setLabel(t('Vocabulary'))
+      ->setDescription(t('The vocabulary to which the term is assigned.'))
+      ->setSetting('target_type', 'taxonomy_vocabulary');
 
     $fields['langcode'] = FieldDefinition::create('language')
       ->setLabel(t('Language code'))

core/modules/taxonomy/lib/Drupal/taxonomy/Tests/TaxonomyTermReferenceItemTest.php

@@ -102,7 +102,7 @@ class TaxonomyTermReferenceItemTest extends FieldUnitTestBase {
     // Make sure the computed term reflects updates to the term id.
     $term2 = entity_create('taxonomy_term', array(
       'name' => $this->randomName(),
-      'vid' => $this->term->vid->value,
+      'vid' => $this->term->bundle(),
       'langcode' => Language::LANGCODE_NOT_SPECIFIED,
     ));
     $term2->save();

core/modules/taxonomy/lib/Drupal/taxonomy/Tests/TermValidationTest.php

@@ -41,6 +41,10 @@ class TermValidationTest extends EntityUnitTestBase {
    * Tests the term validation constraints.
    */
   public function testValidation() {
+    $this->entityManager->getStorageController('taxonomy_vocabulary')->create(array(
+      'vid' => 'tags',
+      'name' => 'Tags',
+    ))->save();
     $term = $this->entityManager->getStorageController('taxonomy_term')->create(array(
       'name' => 'test',
       'vid' => 'tags',