Update to Drupal 8.0.0 beta 14. For more information, see https://drupal.org/node/2544542

core/modules/text/migration_templates/d6_text_settings.yml

@@ -0,0 +1,13 @@
+id: d6_text_settings
+label: Drupal 6 teaser length configuration
+migration_tags:
+  - Drupal 6
+source:
+  plugin: variable
+  variables:
+    - teaser_length
+process:
+  default_summary_length: teaser_length
+destination:
+  plugin: config
+  config_name: text.settings

core/modules/text/src/Tests/Migrate/d6/MigrateTextConfigsTest.php

@@ -0,0 +1,47 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\text\Tests\Migrate\d6\MigrateTextConfigsTest.
+ */
+
+namespace Drupal\text\Tests\Migrate\d6;
+
+use Drupal\config\Tests\SchemaCheckTestTrait;
+use Drupal\migrate_drupal\Tests\d6\MigrateDrupal6TestBase;
+
+/**
+ * Upgrade variables to text.settings.yml.
+ *
+ * @group text
+ */
+class MigrateTextConfigsTest extends MigrateDrupal6TestBase {
+
+  use SchemaCheckTestTrait;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('text');
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->loadDumps(['Variable.php']);
+    $this->executeMigration('d6_text_settings');
+  }
+
+  /**
+   * Tests migration of text variables to text.settings.yml.
+   */
+  public function testTextSettings() {
+    $config = $this->config('text.settings');
+    $this->assertIdentical(456, $config->get('default_summary_length'));
+    $this->assertConfigSchema(\Drupal::service('config.typed'), 'text.settings', $config->get());
+  }
+
+}