Issue #2249149 by ultimike, benjy | chx: Create a system_logging migration.

core/modules/migrate_drupal/config/install/migrate.migration.d6_system_logging.yml

+id: d6_system_logging
+label: Drupal 6 system logging
+source:
+  plugin: variable
+  variables:
+    - error_level
+process:
+  error_level:
+    plugin: static_map
+    source: error_level
+    default_value: all
+    map:
+      0: hide
+      1: some
+      2: all
+      3: verbose
+destination:
+  plugin: config
+  config_name: system.logging

core/modules/migrate_drupal/src/Tests/Dump/Drupal6SystemLogging.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate_drupal\Tests\Dump\Drupal6SystemLogging.
+ */
+
+namespace Drupal\migrate_drupal\Tests\Dump;
+
+/**
+ * Database dump for testing system.logging.yml migration.
+ */
+class Drupal6SystemLogging extends Drupal6DumpBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function load() {
+    $this->createTable('variable');
+    $this->database->insert('variable')->fields(array(
+      'name',
+      'value',
+    ))
+    ->values(array(
+      'name' => 'error_level',
+      'value' => serialize(1),
+    ))
+    ->execute();
+  }
+
+}

core/modules/migrate_drupal/src/Tests/d6/MigrateSystemLoggingTest.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate_drupal\Tests\d6\MigrateSystemLoggingTest.
+ */
+
+namespace Drupal\migrate_drupal\Tests\d6;
+
+use Drupal\migrate\MigrateExecutable;
+use Drupal\config\Tests\SchemaCheckTestTrait;
+use Drupal\migrate_drupal\Tests\MigrateDrupalTestBase;
+
+/**
+ * Tests migration of system error_level variables to configuration.
+ */
+class MigrateSystemLoggingTest extends MigrateDrupalTestBase {
+
+  use SchemaCheckTestTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getInfo() {
+    return array(
+      'name'  => 'Migrate error_level variable to system.logging.yml',
+      'description'  => 'Upgrade error_level variable to system.logging.yml',
+      'group' => 'Migrate Drupal',
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $migration = entity_load('migration', 'd6_system_logging');
+    $dumps = array(
+      $this->getDumpDirectory() . '/Drupal6SystemLogging.php',
+    );
+    $this->prepare($migration, $dumps);
+    $executable = new MigrateExecutable($migration, $this);
+    $executable->import();
+  }
+
+  /**
+   * Tests migration of system error_level variables to system.logging.yml.
+   */
+  public function testSystemLogging() {
+    $config = \Drupal::config('system.logging');
+    $this->assertIdentical($config->get('error_level'), 'some');
+    $this->assertConfigSchema(\Drupal::service('config.typed'), 'system.logging', $config->get());
+  }
+
+}