reverted:
--- b/core/modules/aggregator/src/Plugin/migrate/destination/AggregatorItem.php
+++ /dev/null
@@ -1,42 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\aggregator\Plugin\migrate\destination\AggregatorItem.
- */
-
-namespace Drupal\aggregator\Plugin\migrate\destination;
-
-use Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem;
-use Drupal\migrate\MigrateException;
-use Drupal\migrate\Row;
-use Drupal\migrate\Plugin\migrate\destination\EntityContentBase;
-
-/**
- * @MigrateDestination(
- *   id = "entity:aggregator_item"
- * )
- */
-class AggregatorItem extends EntityContentBase {
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function processStubRow(Row $row) {
-    parent::processStubRow($row);
-    // Work-around for https://www.drupal.org/node/2602662 - this entire class
-    // can be removed when that's committed.
-    if (!$row->getDestinationProperty('fid')) {
-      $field_definitions = \Drupal::entityManager()
-        ->getFieldDefinitions($this->storage->getEntityTypeId(),
-                              $this->getKey('bundle'));
-      $value = EntityReferenceItem::generateSampleValue($field_definitions['fid']);
-      if (is_null($value)) {
-        throw new MigrateException($this->t('Stubbing failed, unable to generate value for field @name',
-          ['@name' => 'fid']));
-      }
-      $row->setDestinationProperty('fid', $value);
-    }
-  }
-
-}
diff -u b/core/modules/aggregator/src/Tests/Migrate/MigrateAggregatorStubTest.php b/core/modules/aggregator/src/Tests/Migrate/MigrateAggregatorStubTest.php
--- b/core/modules/aggregator/src/Tests/Migrate/MigrateAggregatorStubTest.php
+++ b/core/modules/aggregator/src/Tests/Migrate/MigrateAggregatorStubTest.php
@@ -9,6 +9,7 @@
 
 use Drupal\migrate\MigrateException;
 use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
+use Drupal\migrate_drupal\Tests\StubTestTrait;
 
 /**
  * Test stub creation for aggregator feeds and items.
@@ -17,6 +18,8 @@
  */
 class MigrateAggregatorStubTest extends MigrateDrupalTestBase {
 
+  use StubTestTrait;
+
   /**
    * {@inheritdoc}
    */
@@ -45,7 +48,7 @@
     try {
       // We expect an exception, because there's no feed to reference.
       $this->performStubTest('aggregator_item');
-      $this->fail(t('Expected exception has not been thrown.'));
+      $this->fail('Expected exception has not been thrown.');
     }
     catch (MigrateException $e) {
       $this->assertIdentical($e->getMessage(),
diff -u b/core/modules/block_content/src/Tests/Migrate/MigrateBlockContentStubTest.php b/core/modules/block_content/src/Tests/Migrate/MigrateBlockContentStubTest.php
--- b/core/modules/block_content/src/Tests/Migrate/MigrateBlockContentStubTest.php
+++ b/core/modules/block_content/src/Tests/Migrate/MigrateBlockContentStubTest.php
@@ -7,7 +7,9 @@
 
 namespace Drupal\block_content\Tests\Migrate;
 
+use Drupal\block_content\Entity\BlockContentType;
 use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
+use Drupal\migrate_drupal\Tests\StubTestTrait;
 
 /**
  * Test stub creation for block_content entities.
@@ -16,6 +18,8 @@
  */
 class MigrateBlockContentStubTest extends MigrateDrupalTestBase {
 
+  use StubTestTrait;
+
   /**
    * {@inheritdoc}
    */
@@ -30,9 +34,26 @@
   }
 
   /**
-   * Tests creation of block content stubs.
+   * Tests creation of block content stubs with no block_content_type available.
+   */
+  public function testStubFailure() {
+    $entity_id = $this->createStub('block_content');
+    $violations = $this->validateStub('block_content', $entity_id);
+    $this->assertIdentical(count($violations), 1);
+    $this->assertEqual($violations[0]->getMessage(), t('The referenced entity (%type: %id) does not exist.', [
+      '%type' => 'block_content_type',
+      '%id' => 'block_content',
+    ]));
+  }
+
+  /**
+   * Tests creation of block content stubs when there is a block_content_type.
    */
-  public function testStub() {
+  public function testStubSuccess() {
+    BlockContentType::create([
+      'id' => 'test_block_content_type',
+      'label' => 'Test block content type',
+    ])->save();
     $this->performStubTest('block_content');
   }
 
diff -u b/core/modules/comment/src/Tests/Migrate/MigrateCommentStubTest.php b/core/modules/comment/src/Tests/Migrate/MigrateCommentStubTest.php
--- b/core/modules/comment/src/Tests/Migrate/MigrateCommentStubTest.php
+++ b/core/modules/comment/src/Tests/Migrate/MigrateCommentStubTest.php
@@ -7,8 +7,11 @@
 
 namespace Drupal\comment\Tests\Migrate;
 
+use Drupal\comment\Entity\CommentType;
 use Drupal\migrate\MigrateException;
 use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
+use Drupal\migrate_drupal\Tests\StubTestTrait;
+use Drupal\node\Entity\NodeType;
 
 /**
  * Test stub creation for comment entities.
@@ -17,6 +20,8 @@
  */
 class MigrateCommentStubTest extends MigrateDrupalTestBase {
 
+  use StubTestTrait;
+
   /**
    * {@inheritdoc}
    */
@@ -29,6 +34,26 @@
     parent::setUp();
     $this->installEntitySchema('comment');
     $this->installEntitySchema('node');
+    // Make sure uid 0 is created (default uid for comments is 0).
+    $storage = \Drupal::entityManager()->getStorage('user');
+    // Insert a row for the anonymous user.
+    $storage
+      ->create(array(
+        'uid' => 0,
+        'status' => 0,
+        'name' => '',
+      ))
+      ->save();
+    // Need at least one node type and comment type present.
+    NodeType::create([
+      'type' => 'testnodetype',
+      'name' => 'Test node type',
+    ])->save();
+    CommentType::create([
+      'id' => 'testcommenttype',
+      'label' => 'Test comment type',
+      'target_entity_type_id' => 'node',
+    ])->save();
   }
 
   /**
@@ -38,7 +63,7 @@
     try {
       // We expect an exception, because there's no node to reference.
       $this->performStubTest('comment');
-      $this->fail(t('Expected exception has not been thrown.'));
+      $this->fail('Expected exception has not been thrown.');
     }
     catch (MigrateException $e) {
       $this->assertIdentical($e->getMessage(),
diff -u b/core/modules/file/src/Plugin/migrate/destination/EntityFile.php b/core/modules/file/src/Plugin/migrate/destination/EntityFile.php
--- b/core/modules/file/src/Plugin/migrate/destination/EntityFile.php
+++ b/core/modules/file/src/Plugin/migrate/destination/EntityFile.php
@@ -275,17 +275,22 @@
   protected function processStubRow(Row $row) {
     // We stub the uri value ourselves so we can create a real stub file for it.
     if (!$row->getDestinationProperty('uri')) {
+      // @todo Inject the entity manager.
+      // @see https://www.drupal.org/node/2604566
       $field_definitions = \Drupal::entityManager()
         ->getFieldDefinitions($this->storage->getEntityTypeId(),
                               $this->getKey('bundle'));
       $value = UriItem::generateSampleValue($field_definitions['uri']);
-      if (is_null($value)) {
-        throw new MigrateException($this->t('Stubbing failed, unable to generate value for field @name',
-          ['@name' => 'uri']));
+      if (empty($value)) {
+        throw new MigrateException('Stubbing failed, unable to generate value for field uri');
       }
       // generateSampleValue() wraps the value in an array.
       $value = reset($value);
-      // Make it into a proper public file uri.
+      // Make it into a proper public file uri, stripping off the existing
+      // scheme if present.
+      if (strpos($value, 'http://') === 0) {
+        $value = substr($value, 7);
+      }
       $value = 'public://' . substr($value, 9);
       // Create a real file, so File::preSave() can do filesize() on it.
       touch($value);
diff -u b/core/modules/file/src/Tests/Migrate/MigrateFileStubTest.php b/core/modules/file/src/Tests/Migrate/MigrateFileStubTest.php
--- b/core/modules/file/src/Tests/Migrate/MigrateFileStubTest.php
+++ b/core/modules/file/src/Tests/Migrate/MigrateFileStubTest.php
@@ -8,6 +8,7 @@
 namespace Drupal\file\Tests\Migrate;
 
 use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
+use Drupal\migrate_drupal\Tests\StubTestTrait;
 
 /**
  * Test stub creation for file entities.
@@ -16,6 +17,8 @@
  */
 class MigrateFileStubTest extends MigrateDrupalTestBase {
 
+  use StubTestTrait;
+
   /**
    * {@inheritdoc}
    */
diff -u b/core/modules/menu_link_content/src/Tests/Migrate/MigrateMenuLinkContentStubTest.php b/core/modules/menu_link_content/src/Tests/Migrate/MigrateMenuLinkContentStubTest.php
--- b/core/modules/menu_link_content/src/Tests/Migrate/MigrateMenuLinkContentStubTest.php
+++ b/core/modules/menu_link_content/src/Tests/Migrate/MigrateMenuLinkContentStubTest.php
@@ -8,6 +8,7 @@
 namespace Drupal\menu_link_content\Tests\Migrate;
 
 use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
+use Drupal\migrate_drupal\Tests\StubTestTrait;
 
 /**
  * Test stub creation for menu link content entities.
@@ -16,6 +17,8 @@
  */
 class MigrateMenuLinkContentStubTest extends MigrateDrupalTestBase {
 
+  use StubTestTrait;
+
   /**
    * {@inheritdoc}
    */
diff -u b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
--- b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
+++ b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
@@ -8,10 +8,13 @@
 namespace Drupal\migrate\Plugin\migrate\destination;
 
 use Drupal\Component\Plugin\DependentPluginInterface;
+use Drupal\Component\Utility\Random;
+use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Entity\DependencyTrait;
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\Core\Field\FieldItemInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\link\LinkItemInterface;
 use Drupal\migrate\Entity\MigrationInterface;
 use Drupal\migrate\MigrateException;
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
@@ -152,36 +155,60 @@
     }
 
     // Populate any required fields not already populated.
+    // @todo Inject the entity manager.
+    // @see https://www.drupal.org/node/2604566
     $fields = \Drupal::entityManager()
       ->getFieldDefinitions($this->storage->getEntityTypeId(), $bundle_key);
     foreach ($fields as $field_name => $field_definition) {
       if ($field_definition->isRequired() && is_null($row->getDestinationProperty($field_name))) {
         // Use the configured default value for this specific field, if any.
         if ($default_value = $field_definition->getDefaultValueLiteral()) {
-          $value = $default_value;
+          $values[] = $default_value;
         }
         else {
           // Otherwise, ask the field type to generate a sample value.
+          // @todo Inject the field type manager.
+          // @see https://www.drupal.org/node/2604566
+          $field_type = $field_definition->getType();
           /** @var \Drupal\Core\Field\FieldItemInterface $field_type_class */
           $field_type_class = \Drupal::service('plugin.manager.field.field_type')
             ->getPluginClass($field_definition->getType());
-          $value = $field_type_class::generateSampleValue($field_definition);
-          if (is_null($value)) {
+          $values = $field_type_class::generateSampleValue($field_definition);
+          if (is_null($values)) {
             // Handle failure to generate a sample value.
-            switch ($field_definition->getType()) {
-              case 'list_integer':
-                // @todo Remove when https://www.drupal.org/node/2536374 is committed.
-                $value = 1;
-                break;
-              default:
-                throw new MigrateException($this->t('Stubbing failed, unable to generate value for field @name',
-                  ['@name' => $field_name]));
-                break;
+            if ($field_type == 'list_integer') {
+              // @todo Remove when https://www.drupal.org/node/2536374 is committed.
+              $allowed_options = options_allowed_values($field_definition->getFieldStorageDefinition());
+              $values['value'] = array_rand($allowed_options);
+            }
+            else {
+              throw new MigrateException('Stubbing failed, unable to generate value for field ' . $field_name);
+              break;
+            }
+          }
+          else {
+            // @todo UriItem does not generate a valid uri.
+            // @see https://www.drupal.org/node/2605150
+            if ($field_type == 'uri') {
+              foreach ($values as $key => $value) {
+                if (!strpos($value, '://')) {
+                  $suffix_length = Unicode::strlen($value) - 7;
+                  $values[$key] = 'http://' . Unicode::substr($value, 0, $suffix_length);
+                }
+              }
+            }
+            // @todo LinkItem does not generate a valid internal link.
+            // @see https://www.drupal.org/node/2605254
+            elseif ($field_type == 'link') {
+              if ($field_definition->getItemDefinition()->getSetting('link_type') & LinkItemInterface::LINK_INTERNAL) {
+                $random = new Random();
+                $values['uri'] = 'base:' . $random->name(mt_rand(1, 64));
+              }
             }
           }
         }
 
-        $row->setDestinationProperty($field_name, $value);
+        $row->setDestinationProperty($field_name, $values);
       }
     }
   }
reverted:
--- b/core/modules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php
+++ a/core/modules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php
@@ -8,9 +8,6 @@
 namespace Drupal\migrate_drupal\Tests;
 
 use Drupal\Core\Database\Database;
-use Drupal\Core\Entity\ContentEntityBase;
-use Drupal\migrate\Entity\Migration;
-use Drupal\migrate\Row;
 use Drupal\migrate\Tests\MigrateTestBase;
 use Drupal\Component\Plugin\Exception\PluginNotFoundException;
 
@@ -76,47 +73,4 @@
     }
   }
 
-  /**
-   * Test that creating a stub of the given entity type results in a valid
-   * entity.
-   *
-   * @param string $entity_type_id
-   *   The entity type we are stubbing.
-   */
-  protected function performStubTest($entity_type_id) {
-    $destination_id = $this->createStub($entity_type_id);
-    $controller = \Drupal::entityManager()->getStorage($entity_type_id);
-    /** @var \Drupal\Core\Entity\ContentEntityBase $stub_entity */
-    $stub_entity = $controller->load($destination_id);
-    $this->assertTrue($stub_entity, 'Stub successfully created');
-    if ($stub_entity) {
-      $this->assertTrue($stub_entity->validate(), 'Stub is a valid entity');
-    }
-  }
-
-  /**
-   * Create a stub of the given entity type.
-   *
-   * @param string $entity_type_id
-   *   The entity type we are stubbing.
-   *
-   * @return int
-   *   ID of the created entity.
-   */
-  protected function createStub($entity_type_id) {
-    // Create a dummy migration to pass to the destination plugin.
-    $config = [
-      'id' => 'dummy',
-      'migration_tags' => ['Stub test'],
-      'source' => ['plugin' => 'empty'],
-      'process' => [],
-      'destination' => ['plugin' => 'entity:' . $entity_type_id],
-    ];
-    $migration = Migration::create($config);
-    $destination_plugin = $migration->getDestinationPlugin(TRUE);
-    $stub_row = new Row([], [], TRUE);
-    $destination_ids = $destination_plugin->import($stub_row);
-    return reset($destination_ids);
-  }
-
 }
diff -u b/core/modules/node/src/Tests/Migrate/MigrateNodeStubTest.php b/core/modules/node/src/Tests/Migrate/MigrateNodeStubTest.php
--- b/core/modules/node/src/Tests/Migrate/MigrateNodeStubTest.php
+++ b/core/modules/node/src/Tests/Migrate/MigrateNodeStubTest.php
@@ -8,6 +8,8 @@
 namespace Drupal\node\Tests\Migrate;
 
 use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
+use Drupal\migrate_drupal\Tests\StubTestTrait;
+use Drupal\node\Entity\NodeType;
 
 /**
  * Test stub creation for nodes.
@@ -16,6 +18,8 @@
  */
 class MigrateNodeStubTest extends MigrateDrupalTestBase {
 
+  use StubTestTrait;
+
   /**
    * {@inheritdoc}
    */
@@ -27,6 +31,11 @@
   protected function setUp() {
     parent::setUp();
     $this->installEntitySchema('node');
+    // Need at least one node type present.
+    NodeType::create([
+      'type' => 'testnodetype',
+      'name' => 'Test node type',
+    ])->save();
   }
 
   /**
diff -u b/core/modules/shortcut/src/Tests/Migrate/MigrateShortcutStubTest.php b/core/modules/shortcut/src/Tests/Migrate/MigrateShortcutStubTest.php
--- b/core/modules/shortcut/src/Tests/Migrate/MigrateShortcutStubTest.php
+++ b/core/modules/shortcut/src/Tests/Migrate/MigrateShortcutStubTest.php
@@ -8,6 +8,7 @@
 namespace Drupal\shortcut\Tests\Migrate;
 
 use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
+use Drupal\migrate_drupal\Tests\StubTestTrait;
 
 /**
  * Test stub creation for shortcut entities.
@@ -16,6 +17,8 @@
  */
 class MigrateShortcutStubTest extends MigrateDrupalTestBase {
 
+  use StubTestTrait;
+
   /**
    * {@inheritdoc}
    */
diff -u b/core/modules/taxonomy/src/Tests/Migrate/MigrateTaxonomyTermStubTest.php b/core/modules/taxonomy/src/Tests/Migrate/MigrateTaxonomyTermStubTest.php
--- b/core/modules/taxonomy/src/Tests/Migrate/MigrateTaxonomyTermStubTest.php
+++ b/core/modules/taxonomy/src/Tests/Migrate/MigrateTaxonomyTermStubTest.php
@@ -10,7 +10,9 @@
 use Drupal\migrate\Entity\Migration;
 use Drupal\migrate\MigrateExecutable;
 use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
+use Drupal\migrate_drupal\Tests\StubTestTrait;
 use Drupal\taxonomy\Entity\Term;
+use Drupal\taxonomy\Entity\Vocabulary;
 
 /**
  * Test stub creation for taxonomy terms.
@@ -19,6 +21,8 @@
  */
 class MigrateTaxonomyTermStubTest extends MigrateDrupalTestBase {
 
+  use StubTestTrait;
+
   /**
    * {@inheritdoc}
    */
@@ -36,6 +40,10 @@
    * Tests creation of taxonomy term stubs.
    */
   public function testStub() {
+    Vocabulary::create([
+      'vid' => 'test_vocabulary',
+      'name' => 'Test vocabulary',
+    ])->save();
     $this->performStubTest('taxonomy_term');
   }
 
@@ -104,7 +112,7 @@
     $stub_entity = Term::load(2);
     $this->assertTrue($stub_entity, 'Stub successfully created');
     if ($stub_entity) {
-      $this->assertTrue($stub_entity->validate(), 'Stub is a valid entity');
+      $this->assertIdentical(count($stub_entity->validate()), 0, 'Stub is a valid entity');
     }
   }
 }
diff -u b/core/modules/user/src/Plugin/migrate/destination/EntityUser.php b/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
--- b/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
+++ b/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
@@ -10,6 +10,7 @@
 use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Field\Plugin\Field\FieldType\EmailItem;
 use Drupal\Core\Password\PasswordInterface;
 use Drupal\migrate\Entity\MigrationInterface;
 use Drupal\migrate\MigrateException;
@@ -104,8 +105,18 @@
    */
   protected function processStubRow(Row $row) {
     parent::processStubRow($row);
-    // Work-around for https://www.drupal.org/node/2602066 - this function can
-    // be removed when that's committed.
+    // Email address is not defined as required in the base field definition but
+    // is effectively required by the UserMailRequired constraint. This means
+    // that Entity::processStubRow() did not populate it - we do it here.
+    // @todo Inject the entity manager.
+    // @see https://www.drupal.org/node/2604566
+    $field_definitions = \Drupal::entityManager()
+      ->getFieldDefinitions($this->storage->getEntityTypeId(),
+                            $this->getKey('bundle'));
+    $mail = EmailItem::generateSampleValue($field_definitions['mail']);
+    $row->setDestinationProperty('mail', reset($mail));
+
+    // %todo Work-around for https://www.drupal.org/node/2602066.
     $name = $row->getDestinationProperty('name');
     if (is_array($name)) {
       $name = reset($name);
diff -u b/core/modules/user/src/Tests/Migrate/MigrateUserStubTest.php b/core/modules/user/src/Tests/Migrate/MigrateUserStubTest.php
--- b/core/modules/user/src/Tests/Migrate/MigrateUserStubTest.php
+++ b/core/modules/user/src/Tests/Migrate/MigrateUserStubTest.php
@@ -8,6 +8,7 @@
 namespace Drupal\user\Tests\Migrate;
 
 use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
+use Drupal\migrate_drupal\Tests\StubTestTrait;
 
 /**
  * Test stub creation for user entities.
@@ -16,6 +17,8 @@
  */
 class MigrateUserStubTest extends MigrateDrupalTestBase {
 
+  use StubTestTrait;
+
   /**
    * {@inheritdoc}
    */
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate_drupal/src/Tests/StubTestTrait.php
@@ -0,0 +1,80 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate_drupal\Tests\StubTestTrait.
+ */
+
+namespace Drupal\migrate_drupal\Tests;
+use Drupal\migrate\Entity\Migration;
+use Drupal\migrate\Row;
+
+/**
+ * Provides common functionality for testing stubbing.
+ */
+trait StubTestTrait {
+
+  /**
+   * Test that creating a stub of the given entity type results in a valid
+   * entity.
+   *
+   * @param string $entity_type_id
+   *   The entity type we are stubbing.
+   */
+  protected function performStubTest($entity_type_id) {
+    $entity_id = $this->createStub($entity_type_id);
+    $this->assertTrue($entity_id, 'Stub successfully created');
+    if ($entity_id) {
+      $violations = $this->validateStub($entity_type_id, $entity_id);
+      if (!$this->assertIdentical(count($violations), 0, 'Stub is a valid entity')) {
+        foreach ($violations as $violation) {
+          $this->fail((string)$violation->getMessage());
+        }
+      }
+    }
+  }
+
+  /**
+   * Create a stub of the given entity type.
+   *
+   * @param string $entity_type_id
+   *   The entity type we are stubbing.
+   *
+   * @return int
+   *   ID of the created entity.
+   */
+  protected function createStub($entity_type_id) {
+    // Create a dummy migration to pass to the destination plugin.
+    $config = [
+      'id' => 'dummy',
+      'migration_tags' => ['Stub test'],
+      'source' => ['plugin' => 'empty'],
+      'process' => [],
+      'destination' => ['plugin' => 'entity:' . $entity_type_id],
+    ];
+    $migration = Migration::create($config);
+    $destination_plugin = $migration->getDestinationPlugin(TRUE);
+    $stub_row = new Row([], [], TRUE);
+    $destination_ids = $destination_plugin->import($stub_row);
+    return reset($destination_ids);
+  }
+
+  /**
+   * Perform validation on a stub entity.
+   *
+   * @param string $entity_type_id
+   *   The entity type we are stubbing.
+   * @param string $entity_id
+   *   ID of the stubbed entity to validate.
+   *
+   * @return \Drupal\Core\Entity\EntityConstraintViolationListInterface
+   *   List of constraint violations identified.
+   */
+  protected function validateStub($entity_type_id, $entity_id) {
+    $controller = \Drupal::entityManager()->getStorage($entity_type_id);
+    /** @var \Drupal\Core\Entity\ContentEntityInterface $stub_entity */
+    $stub_entity = $controller->load($entity_id);
+    return $stub_entity->validate();
+  }
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Field/FieldGenerateSampleValueTest.php
@@ -0,0 +1,63 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Field\FieldGenerateSampleValueTest.
+ */
+
+namespace Drupal\Tests\Core\Field;
+
+use Drupal\Core\Field\Plugin\Field\FieldType\BooleanItem;
+use Drupal\Core\Field\TypedData\FieldItemDataDefinition;
+use Drupal\Core\TypedData\DataDefinition;
+use Drupal\Core\TypedData\Plugin\DataType\BooleanData;
+use Drupal\Core\TypedData\Plugin\DataType\FloatData;
+use Drupal\Core\TypedData\Plugin\DataType\IntegerData;
+use Drupal\Core\TypedData\Plugin\DataType\StringData;
+use Drupal\Core\TypedData\Plugin\DataType\Uri;
+use Drupal\Core\TypedData\PrimitiveInterface;
+use Drupal\Core\Validation\Plugin\Validation\Constraint\PrimitiveTypeConstraint;
+use Drupal\Core\Validation\Plugin\Validation\Constraint\PrimitiveTypeConstraintValidator;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass Drupal\Core\Validation\Plugin\Validation\Constraint\PrimitiveTypeConstraintValidator
+ * @group field
+ */
+class FieldGenerateSampleValueTest extends UnitTestCase {
+
+  /**
+   * @covers ::validate
+   *
+   * @dataProvider provideTestValidate
+   */
+  public function testValidate(PrimitiveInterface $typed_data, $value) {
+    $context = $this->getMock('\Symfony\Component\Validator\Context\ExecutionContextInterface');
+    $context->expects($this->any())
+      ->method('getObject')
+      ->willReturn($typed_data);
+
+    $context->expects($this->never())
+      ->method('addViolation');
+
+    $constraint = new PrimitiveTypeConstraint();
+
+    $validate = new PrimitiveTypeConstraintValidator();
+    $validate->initialize($context);
+    $validate->validate($value, $constraint);
+  }
+
+  public function provideTestValidate() {
+    $data = [];
+    $field_definition = FieldItemDataDefinition::createFromDataType('field_item:boolean');
+    $data[] = [new BooleanData(DataDefinition::create('boolean')), BooleanItem::generateSampleValue($field_definition->getFieldDefinition())];
+    $data[] = [new FloatData(DataDefinition::create('float')), 'test'];
+    $data[] = [new IntegerData(DataDefinition::create('integer')), 'test'];
+    $data[] = [new StringData(DataDefinition::create('string')), []];
+    $data[] = [new Uri(DataDefinition::create('uri')), 'drupal.org'];
+
+    return $data;
+  }
+
+}