only in patch2:
unchanged:
--- a/core/modules/file/src/Plugin/migrate/destination/EntityFile.php
+++ b/core/modules/file/src/Plugin/migrate/destination/EntityFile.php
@@ -9,6 +9,7 @@
 
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Field\Plugin\Field\FieldType\UriItem;
 use Drupal\Core\File\FileSystemInterface;
 use Drupal\Core\StreamWrapper\LocalStream;
 use Drupal\Core\StreamWrapper\StreamWrapperManagerInterface;
@@ -77,6 +78,12 @@ public static function create(ContainerInterface $container, array $configuratio
    * {@inheritdoc}
    */
   protected function getEntity(Row $row, array $old_destination_id_values) {
+    // For stub rows, there is no real file to deal with, let the stubbing
+    // process take its default path.
+    if ($row->isStub()) {
+      return parent::getEntity($row, $old_destination_id_values);
+    }
+
     $destination = $row->getDestinationProperty($this->configuration['destination_path_property']);
     $entity = $this->storage->loadByProperties(['uri' => $destination]);
     if ($entity) {
@@ -91,6 +98,12 @@ protected function getEntity(Row $row, array $old_destination_id_values) {
    * {@inheritdoc}
    */
   public function import(Row $row, array $old_destination_id_values = array()) {
+    // For stub rows, there is no real file to deal with, let the stubbing
+    // process create the stub entity.
+    if ($row->isStub()) {
+      return parent::import($row, $old_destination_id_values);
+    }
+
     $file = $row->getSourceProperty($this->configuration['source_path_property']);
     $destination = $row->getDestinationProperty($this->configuration['destination_path_property']);
     $source = $this->configuration['source_base_path'] . $file;
@@ -256,4 +269,30 @@ protected function urlencode($filename) {
     return $filename;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function processStubRow(Row $row) {
+    parent::processStubRow($row);
+    // Work-around for https://www.drupal.org/node/2603726 - this function can
+    // be removed when that's committed.
+    if (!$row->getDestinationProperty('uri')) {
+      $field_definitions = \Drupal::entityManager()
+        ->getFieldDefinitions($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']));
+      }
+      // generateSampleValue() wraps the value in an array.
+      $value = reset($value);
+      // Make it into a proper public file uri.
+      $value = 'public://' . substr($value, 9);
+      // Create a real file, so File::preSave() can do filesize() on it.
+      touch($value);
+      $row->setDestinationProperty('uri', $value);
+    }
+  }
+
 }