Update to Drupal 8.2.2. For more information, see https://www.drupal.org/project/drupal/releases/8.2.2

core/modules/user/src/Entity/User.php

@@ -416,6 +416,9 @@ class User extends ContentEntityBase implements UserInterface {
       static::$anonymousUser = new $class([
         'uid' => [LanguageInterface::LANGCODE_DEFAULT => 0],
         'name' => [LanguageInterface::LANGCODE_DEFAULT => ''],
+        // Explicitly set the langcode to ensure that field definitions do not
+        // need to be fetched to figure out a default.
+        'langcode' => [LanguageInterface::LANGCODE_DEFAULT => LanguageInterface::LANGCODE_NOT_SPECIFIED]
       ], $entity_type->id());
     }
     return clone static::$anonymousUser;

core/modules/user/src/Plugin/migrate/process/UserLangcode.php

@@ -0,0 +1,86 @@
+<?php
+
+namespace Drupal\user\Plugin\migrate\process;
+
+use Drupal\Core\Language\LanguageManager;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a process plugin for the user langcode.
+ *
+ * @MigrateProcessPlugin(
+ *   id = "user_langcode"
+ * )
+ */
+class UserLangcode extends ProcessPluginBase implements ContainerFactoryPluginInterface {
+
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManager
+   */
+  protected $languageManager;
+
+  /**
+   * Constructs a UserLangcode object.
+   *
+   * @param array $configuration
+   *   Plugin configuration.
+   * @param string $plugin_id
+   *   The plugin ID.
+   * @param mixed $plugin_definition
+   *   The plugin definiiton.
+   * @param \Drupal\Core\Language\LanguageManager $language_manager
+   *   The language manager service.
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, LanguageManager $language_manager) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+    $this->languageManager = $language_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('language_manager')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    if (!isset($this->configuration['fallback_to_site_default'])) {
+      $this->configuration['fallback_to_site_default'] = TRUE;
+    }
+
+    // If the user's language is empty, it means the locale module was not
+    // installed, so the user's langcode should be English and the user's
+    // preferred_langcode and preferred_admin_langcode should fallback to the
+    // default language.
+    if (empty($value)) {
+      if ($this->configuration['fallback_to_site_default']) {
+        return $this->languageManager->getDefaultLanguage()->getId();
+      }
+      else {
+        return 'en';
+      }
+    }
+    // If the user's language does not exists, use the default language.
+    elseif ($this->languageManager->getLanguage($value) === NULL) {
+      return $this->languageManager->getDefaultLanguage()->getId();
+    }
+
+    // If the langcode is a valid one, just return it.
+    return $value;
+  }
+
+}

core/modules/user/src/Plugin/migrate/source/d6/ProfileFieldValues.php

@@ -21,7 +21,7 @@ class ProfileFieldValues extends DrupalSqlBase {
   public function query() {
     $query = $this->select('profile_values', 'pv')
       ->distinct()
-      ->fields('pv', array('fid', 'uid'));
+      ->fields('pv', array('uid'));
 
     return $query;
   }

core/modules/user/src/Plugin/migrate/source/d6/Role.php

@@ -69,7 +69,15 @@ class Role extends DrupalSqlBase {
       ->condition('rid', $rid)
       ->execute()
       ->fetchField();
-    $row->setSourceProperty('permissions', explode(', ', $permissions));
+
+    // If a role has no permissions then set to an empty array. The role will
+    // be migrated and given the default D8 permissions.
+    if ($permissions) {
+      $row->setSourceProperty('permissions', explode(', ', $permissions));
+    }
+    else {
+      $row->setSourceProperty('permissions', []);
+    }
     if (isset($this->filterPermissions[$rid])) {
       $row->setSourceProperty("filter_permissions:$rid", $this->filterPermissions[$rid]);
     }

core/modules/user/src/Plugin/migrate/source/d6/User.php

@@ -100,6 +100,7 @@ class User extends DrupalSqlBase {
       'name' => $this->t('Username'),
       'pass' => $this->t('Password'),
       'mail' => $this->t('Email address'),
+      'theme' => $this->t('Theme'),
       'signature' => $this->t('Signature'),
       'signature_format' => $this->t('Signature format'),
       'created' => $this->t('Registered timestamp'),