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
@@ -7,6 +7,7 @@
 
 namespace Drupal\aggregator\Tests\Migrate;
 
+use Drupal\migrate\MigrateException;
 use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
 
 /**
@@ -41,6 +42,18 @@
    * Tests creation of aggregator feed items.
    */
   public function testItemStub() {
+    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.'));
+    }
+    catch (MigrateException $e) {
+      $this->assertIdentical($e->getMessage(),
+        'Stubbing failed, unable to generate value for field fid');
+    }
+
+    // The stub should pass when there's a feed to point to.
+    $this->createStub('aggregator_feed');
     $this->performStubTest('aggregator_item');
   }
 
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
@@ -7,6 +7,7 @@
 
 namespace Drupal\comment\Tests\Migrate;
 
+use Drupal\migrate\MigrateException;
 use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
 
 /**
@@ -34,7 +35,18 @@
    * Tests creation of comment stubs.
    */
   public function testStub() {
-    // @todo: We need to create a node for the comment stub to attach to.
+    try {
+      // We expect an exception, because there's no node to reference.
+      $this->performStubTest('comment');
+      $this->fail(t('Expected exception has not been thrown.'));
+    }
+    catch (MigrateException $e) {
+      $this->assertIdentical($e->getMessage(),
+        'Stubbing failed, unable to generate value for field entity_id');
+    }
+
+    // The stub should pass when there's a node to point to.
+    $this->createStub('node');
     $this->performStubTest('comment');
   }
 
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
@@ -13,6 +13,8 @@
 use Drupal\Core\Field\FieldItemInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\migrate\Entity\MigrationInterface;
+use Drupal\migrate\MigrateException;
+use Drupal\migrate\Plugin\MigrateIdMapInterface;
 use Drupal\migrate\Row;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
@@ -165,14 +167,16 @@
             ->getPluginClass($field_definition->getType());
           $value = $field_type_class::generateSampleValue($field_definition);
           if (is_null($value)) {
-            // Not all field types provide sample values. Do the best we can.
+            // Handle failure to generate a sample value.
             switch ($field_definition->getType()) {
-              case 'entity_reference':
-                // @todo Look for an existing entity of the appropriate type.
-                break;
               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' => $field_name]));
+                break;
             }
           }
         }
diff -u b/core/modules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php b/core/modules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php
--- b/core/modules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php
+++ b/core/modules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php
@@ -84,6 +84,26 @@
    *   The entity type we are stubbing.
    */
   protected function performStubTest($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',
@@ -96,13 +116,7 @@
     $destination_plugin = $migration->getDestinationPlugin(TRUE);
     $stub_row = new Row([], [], TRUE);
     $destination_ids = $destination_plugin->import($stub_row);
-    $controller = \Drupal::entityManager()->getStorage($entity_type_id);
-    /** @var \Drupal\Core\Entity\ContentEntityBase $stub_entity */
-    $stub_entity = $controller->load(reset($destination_ids));
-    $this->assertTrue($stub_entity, 'Stub successfully created');
-    if ($stub_entity) {
-      $this->assertTrue($stub_entity->validate(), 'Stub is a valid entity');
-    }
+    return reset($destination_ids);
   }
 
 }
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
@@ -27,10 +27,12 @@
   protected function setUp() {
     parent::setUp();
     $this->installEntitySchema('shortcut');
+    // Make sure the 'default' shortcut_set is installed.
+    $this->installConfig(['shortcut']);
   }
 
   /**
-   * Tests creation of menu link content stubs.
+   * Tests creation of shortcut stubs.
    */
   public function testStub() {
     $this->performStubTest('shortcut');
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/aggregator/src/Plugin/migrate/destination/AggregatorItem.php
@@ -0,0 +1,42 @@
+<?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);
+    }
+  }
+
+}
only in patch2:
unchanged:
--- a/core/modules/comment/src/Plugin/migrate/destination/EntityComment.php
+++ b/core/modules/comment/src/Plugin/migrate/destination/EntityComment.php
@@ -110,32 +110,9 @@ public function import(Row $row, array $old_destination_id_values = array()) {
    */
   protected function processStubRow(Row $row) {
     parent::processStubRow($row);
-    $stub_commented_entity_type = $row->getDestinationProperty('entity_type');
-
-    // While parent::getEntity() fills the bundle property for stub entities
-    // if it's still empty, here we must also make sure entity_id/entity_type
-    // are filled (so $comment->getCommentedEntity() always returns a value).
-    if (empty($this->stubCommentedEntityIds[$stub_commented_entity_type])) {
-      // Fill stub entity id. Any id will do, as long as it exists.
-      $entity_type = $this->entityManager->getDefinition($stub_commented_entity_type);
-      $id_key = $entity_type->getKey('id');
-      $result = $this->entityQuery
-        ->get($stub_commented_entity_type)
-        ->range(0, 1)
-        ->execute();
-      if ($result) {
-        $this->stubCommentedEntityIds[$stub_commented_entity_type] = array_pop($result);
-        $row->setSourceProperty($id_key, $this->stubCommentedEntityIds[$stub_commented_entity_type]);
-      }
-      else {
-        throw new MigrateException(t('Could not find parent entity to use for comment %id', ['%id' => implode(':', $row->getSourceIdValues())]), MigrationInterface::MESSAGE_ERROR);
-      }
-    }
-
-    $row->setDestinationProperty('entity_id', $this->stubCommentedEntityIds[$stub_commented_entity_type]);
-    $row->setDestinationProperty('entity_type', $stub_commented_entity_type);
-    $row->setDestinationProperty('created', REQUEST_TIME);
-    $row->setDestinationProperty('changed', REQUEST_TIME);
+    // Neither uid nor name is required in itself, but it is required to set one
+    // of them.
+    $row->setDestinationProperty('name', 'anonymous_stub');
   }
 
 }
only in patch2:
unchanged:
--- a/core/modules/taxonomy/src/Plugin/migrate/destination/EntityTaxonomyTerm.php
+++ /dev/null
@@ -1,30 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\taxonomy\Plugin\migrate\destination\EntityTaxonomyTerm.
- */
-
-namespace Drupal\taxonomy\Plugin\migrate\destination;
-
-use Drupal\migrate\Row;
-use Drupal\migrate\Plugin\migrate\destination\EntityContentBase;
-
-/**
- * @MigrateDestination(
- *   id = "entity:taxonomy_term"
- * )
- */
-class EntityTaxonomyTerm extends EntityContentBase {
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function getEntity(Row $row, array $old_destination_id_values) {
-    if ($row->isStub()) {
-      $row->setDestinationProperty('name', $this->t('Stub name for source tid:') . $row->getSourceProperty('tid'));
-    }
-    return parent::getEntity($row, $old_destination_id_values);
-  }
-
-}
only in patch2:
unchanged:
--- a/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
+++ b/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\user\Plugin\migrate\destination;
 
+use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\Core\Password\PasswordInterface;
@@ -98,4 +99,20 @@ public function import(Row $row, array $old_destination_id_values = array()) {
     return $ids;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  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.
+    $name = $row->getDestinationProperty('name');
+    if (is_array($name)) {
+      $name = reset($name);
+    }
+    if (Unicode::strlen($name) > USERNAME_MAX_LENGTH) {
+      $row->setDestinationProperty('name', Unicode::substr($name, 0, USERNAME_MAX_LENGTH));
+    }
+  }
+
 }