diff -u b/core/modules/migrate/config/migrate.migration.d6_system_site.yml b/core/modules/migrate/config/migrate.migration.d6_system_site.yml
--- b/core/modules/migrate/config/migrate.migration.d6_system_site.yml
+++ b/core/modules/migrate/config/migrate.migration.d6_system_site.yml
@@ -11,7 +11,6 @@
         - drupal_weight_select_max
         - admin_compact_mode
 process:
-    plugin: copy_from_source
     name: site_name
     mail: site_mail
     slogan: site_slogan
diff -u b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php
--- b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php
@@ -10,7 +10,6 @@
 use Drupal\Core\Config\Entity\ConfigEntityBase;
 use Drupal\migrate\MigrateException;
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
-use Drupal\migrate\Plugin\MigrateProcessBag;
 
 /**
  * Defines the Migration entity.
@@ -95,20 +94,11 @@
   /**
    * The configuration describing the process plugins.
    *
-   * Used to initialize $migrateProcessBag.
-   *
    * @var array
    */
   public $process;
 
   /**
-   * The array which stores all active process plugins.
-   *
-   * @var array
-   */
-  protected $processPlugins = array();
-
-  /**
    * The destination configuration, with at least a 'plugin' key.
    *
    * Used to initialize $destinationPlugin.
@@ -144,7 +134,7 @@
    * The source identifiers.
    *
    * An array of source identifiers: the keys are the name of the properties,
-   * the values are dependent on the id map plugin.
+   * the values are dependent on the ID map plugin.
    *
    * @var array
    */
@@ -154,7 +144,7 @@
    * The destination identifiers.
    *
    * An array of destination identifiers: the keys are the name of the
-   * properties, the values are dependent on the id map plugin.
+   * properties, the values are dependent on the ID map plugin.
    *
    * @var array
    */
@@ -200,7 +190,7 @@
   /**
    * {@inheritdoc}
    */
-  public function getSource() {
+  public function getSourcePlugin() {
     if (!isset($this->sourcePlugin)) {
       $this->sourcePlugin = \Drupal::service('plugin.manager.migrate.source')->createInstance($this->source['plugin'], $this->source, $this);
     }
@@ -210,23 +200,27 @@
   /**
    * {@inheritdoc}
    */
-  public function getProcess() {
-    foreach ($this->getProcessNormalized() as $property => $configurations) {
+  public function getProcessPlugins(array $process = NULL) {
+    if (!isset($process)) {
+      $process = $this->process;
+    }
+    $process_plugins = array();
+    foreach ($this->getProcessNormalized($process) as $property => $configurations) {
       foreach ($configurations as $configuration) {
-        $this->processPlugins[$property] = array();
+        $process_plugins[$property] = array();
         if (isset($configuration['source'])) {
-          $this->processPlugins[$property][] = \Drupal::service('plugin.manager.migrate.process')->createInstance('get', $configuration, $this);
+          $process_plugins[$property][] = \Drupal::service('plugin.manager.migrate.process')->createInstance('get', $configuration, $this);
         }
         // Get is already handled.
         if ($configuration['plugin'] != 'get') {
-          $this->processPlugins[$property][] = \Drupal::service('plugin.manager.migrate.process')->createInstance($configuration['plugin'], $configuration, $this);
+          $process_plugins[$property][] = \Drupal::service('plugin.manager.migrate.process')->createInstance($configuration['plugin'], $configuration, $this);
         }
-        if (!$this->processPlugins[$property]) {
+        if (!$process_plugins[$property]) {
           throw new MigrateException("Invalid process configuration for $property");
         }
       }
     }
-    return $this->processPlugins;
+    return $process_plugins;
   }
 
   /**
@@ -235,9 +229,9 @@
    * @return array
    *   The normalized process configuration.
    */
-  protected function getProcessNormalized() {
+  protected function getProcessNormalized($process) {
     $normalized_configurations = array();
-    foreach ($this->process as $destination => $configuration) {
+    foreach ($process as $destination => $configuration) {
       if (is_string($configuration)) {
         $configuration = array(
           'plugin' => 'get',
@@ -255,7 +249,7 @@
   /**
    * {@inheritdoc}
    */
-  public function getDestination() {
+  public function getDestinationPlugin() {
     if (!isset($this->destinationPlugin)) {
       $this->destinationPlugin = \Drupal::service('plugin.manager.migrate.destination')->createInstance($this->destination['plugin'], $this->destination, $this);
     }
diff -u b/core/modules/migrate/lib/Drupal/migrate/Entity/MigrationInterface.php b/core/modules/migrate/lib/Drupal/migrate/Entity/MigrationInterface.php
--- b/core/modules/migrate/lib/Drupal/migrate/Entity/MigrationInterface.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Entity/MigrationInterface.php
@@ -55,21 +55,24 @@
    *
    * @return \Drupal\migrate\Plugin\MigrateSourceInterface
    */
-  public function getSource();
+  public function getSourcePlugin();
 
   /**
    * Returns the process plugins.
    *
+   * @param array $process
+   *   A process configuration array.
    * @return array
+   *   A list of process plugins.
    */
-  public function getProcess();
+  public function getProcessPlugins(array $process = NULL);
 
   /**
    * Returns the initialized destination plugin.
    *
    * @return \Drupal\migrate\Plugin\MigrateDestinationInterface
    */
-  public function getDestination();
+  public function getDestinationPlugin();
 
   /**
    * Returns the initialized id_map plugin.
diff -u b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php
--- b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php
+++ b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php
@@ -210,7 +210,7 @@
   public function import() {
     $return = MigrationInterface::RESULT_COMPLETED;
     $source = $this->getSource();
-    $destination = $this->migration->getDestination();
+    $destination = $this->migration->getDestinationPlugin();
     $id_map = $this->migration->getIdMap();
 
     try {
@@ -222,7 +222,7 @@
           array('!e' => $e->getMessage())));
       return MigrationInterface::RESULT_FAILED;
     }
-    while ($this->getSource()->valid()) {
+    while ($source->valid()) {
       $row = $source->current();
       $this->sourceIdValues = $row->getSourceIdValues();
 
@@ -234,7 +234,7 @@
 
       try {
         $destination_id_values = $destination->import($row);
-        // @TODO handle the successful but no id case like config.
+        // @TODO handle the successful but no ID case like config.
         if ($destination_id_values) {
           $id_map->saveIdMapping($row, $destination_id_values, $this->needsUpdate, $this->rollbackAction);
           $this->successes_since_feedback++;
@@ -302,9 +302,9 @@
   /**
    * Apply transformations to a data row received from the source.
    */
-  protected function processRow(Row $row) {
+  public function processRow(Row $row, array $process = NULL) {
     $value = NULL;
-    foreach ($this->migration->getProcess() as $destination => $plugins) {
+    foreach ($this->migration->getProcessPlugins($process) as $destination => $plugins) {
       foreach ($plugins as $plugin) {
         $value = $plugin->transform($value, $this, $row, $destination);
       }
diff -u b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateDestinationInterface.php b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateDestinationInterface.php
--- b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateDestinationInterface.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateDestinationInterface.php
@@ -51,12 +51,12 @@
 
   /**
    * Derived classes must implement import(), to construct one new object (pre-populated
-   * using id mappings in the Migration).
+   * using ID mappings in the Migration).
    */
   public function import(Row $row);
 
   /**
-   * Delete the specified ids from the target Drupal.
+   * Delete the specified IDs from the target Drupal.
    * @param array $destination_identifiers
    */
   public function rollbackMultiple(array $destination_identifiers);
diff -u b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateIdMapInterface.php b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateIdMapInterface.php
--- b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateIdMapInterface.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateIdMapInterface.php
@@ -13,9 +13,9 @@
 use Drupal\migrate\Row;
 
 /**
- * An interface for migrate id mappings.
+ * An interface for migrate ID mappings.
  *
- * Migrate id mappings maintain a relation between source ID and
+ * Migrate ID mappings maintain a relation between source ID and
  * destination ID for audit and rollback purposes.
  */
 interface MigrateIdMapInterface extends PluginInspectionInterface {
reverted:
--- b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateProcessBag.php
+++ /dev/null
@@ -1,72 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\migrate\Plugin\FieldMappingBag.
- */
-
-namespace Drupal\migrate\Plugin;
-
-use Drupal\Component\Plugin\DefaultPluginBag;
-use Drupal\Component\Plugin\Exception\UnknownPluginException;
-use Drupal\Component\Plugin\PluginManagerInterface;
-use Drupal\migrate\Entity\MigrationInterface;
-
-/**
- * Contains all active process plugins on a migration.
- */
-class MigrateProcessBag extends DefaultPluginBag {
-
-  /**
-   * The migration entity this process bag is attached to.
-   *
-   * @var \Drupal\migrate\Entity\MigrationInterface
-   */
-  protected $migration;
-
-  /**
-   * The key within the plugin configuration that contains the plugin ID.
-   *
-   * @var string
-   */
-  protected $pluginKey = 'plugin';
-
-  /**
-   * @var \Drupal\migrate\Plugin\MigratePluginManager
-   */
-  protected $manager;
-
-  /**
-   * Constructs a new MigrateProcessBag.
-   *
-   * @param \Drupal\Component\Plugin\PluginManagerInterface $manager
-   *   The plugin manager to initialize process plugins.
-   * @param array $configurations
-   *   (optional) An associative array containing the initial configuration for
-   *   each plugin in the bag, keyed by plugin instance ID.
-   * @param \Drupal\migrate\Entity\MigrationInterface $migration
-   *   The migration entity this process bag is attached to.
-   */
-  public function __construct(PluginManagerInterface $manager, array $configurations = array(), MigrationInterface $migration) {
-    parent::__construct($manager, $configurations);
-    $this->migration = $migration;
-  }
-
-  /**
-   * {@inheritdoc}
-   *
-   * Extends in order to pass the migration entity to the plugin manager.
-   */
-  public function initializePlugin($instance_id) {
-    $this->configurations[$instance_id] += array('plugin' => 'copy_from_source');
-    $configuration = isset($this->configurations[$instance_id]) ? $this->configurations[$instance_id] : array();
-    if (!isset($configuration[$this->pluginKey])) {
-      throw new UnknownPluginException($instance_id);
-    }
-    $plugin_key = $configuration[$this->pluginKey];
-    unset($configuration[$this->pluginKey]);
-    $this->pluginInstances[$instance_id] = $this->manager->createInstance($plugin_key, $configuration, $this->migration);
-    $this->addInstanceID($instance_id);
-  }
-
-}
diff -u b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/SourcePluginBase.php b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/SourcePluginBase.php
--- b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/SourcePluginBase.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/SourcePluginBase.php
@@ -47,6 +47,7 @@
    */
   public function prepareRow(Row $row) {
     $this->getModuleHandler()->invokeAll('migrate_prepare_row', $row, $this, $this->migration);
+    $this->getModuleHandler()->invokeAll('migrate_ '. $this->migration->id() . '_prepare_row', $row, $this, $this->migration);
     return TRUE;
   }
 
diff -u b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/SqlBase.php b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/SqlBase.php
--- b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/SqlBase.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/SqlBase.php
@@ -70,6 +70,11 @@
     return Database::getConnection('default', $key);
   }
 
+  protected function select($table, $alias = NULL, array $options = array()) {
+    $options['fetch'] = \PDO::FETCH_ASSOC;
+    return $this->getDatabase()->select($table, $alias, $options);
+  }
+
   /**
    * Implementation of MigrateSource::performRewind().
    *
@@ -160,7 +165,7 @@
       }
     }
 
-    return $this->query->execute();
+    return new \IteratorIterator($this->query->execute());
   }
 
   /**
diff -u b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/d6/Drupal6SqlBase.php b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/d6/Drupal6SqlBase.php
--- b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/d6/Drupal6SqlBase.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/source/d6/Drupal6SqlBase.php
@@ -18,10 +18,10 @@
 abstract class Drupal6SqlBase extends SqlBase {
 
   /**
-   * Retrieves all system data informationfrom origin system.
+   * Retrieves all system data information from origin system.
    *
    * @return array
-   *   List of system table informationi keyed by type and name.
+   *   List of system table information keyed by type and name.
    */
   public function getSystemData() {
     static $system_data;
@@ -68,2 +68,15 @@
 
+  protected function variableGet($name, $default) {
+    try {
+      $result = $this->database
+        ->query('SELECT value FROM {variable} WHERE name = :name', array(':name' => $name))
+        ->fetchField();
+    }
+    // The table might not exist.
+    catch (\Exception $e) {
+      $result = FALSE;
+    }
+    return $result !== FALSE ? unserialize($result) : $default;
+  }
+
 }
diff -u b/core/modules/migrate/lib/Drupal/migrate/Row.php b/core/modules/migrate/lib/Drupal/migrate/Row.php
--- b/core/modules/migrate/lib/Drupal/migrate/Row.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Row.php
@@ -62,14 +62,14 @@
    * @param array $values
    *   An array of values to add as properties on the object.
    * @param array $source_ids
-   *   An array containing the ids of the source using the keys as the field
+   *   An array containing the IDs of the source using the keys as the field
    *   names.
    * @param array $destination_ids
-   *   An array containing the ids of the destination using the keys as the field
+   *   An array containing the IDs of the destination using the keys as the field
    *   names.
    *
    * @throws \InvalidArgumentException
-   *   Thrown when a source id property does not exist.
+   *   Thrown when a source ID property does not exist.
    */
   public function __construct(array $values, array $source_ids) {
     $this->source = $values;
@@ -196,7 +196,7 @@
   }
 
   /**
-   * Sets the Migrate id mappings.
+   * Sets the Migrate ID mappings.
    *
    * @param array $id_map
    *   An array of mappings between source ID and destination ID.
@@ -206,7 +206,7 @@
   }
 
   /**
-   * Retrieves the Migrate id mappings.
+   * Retrieves the Migrate ID mappings.
    *
    * @return array
    *   An array of mapping between source and destination identifiers.
@@ -224,7 +224,7 @@
   }
 
   /**
-   * Checks whether the row has changed compared to the original id map.
+   * Checks whether the row has changed compared to the original ID map.
    *
    * return bool
    *   TRUE if the row has changed, FALSE otherwise. If setIdMap() was not
diff -u b/core/modules/migrate/lib/Drupal/migrate/Source.php b/core/modules/migrate/lib/Drupal/migrate/Source.php
--- b/core/modules/migrate/lib/Drupal/migrate/Source.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Source.php
@@ -151,7 +151,7 @@
     if ($this->skipCount) {
       return -1;
     }
-    $source = $this->migration->getSource();
+    $source = $this->migration->getSourcePlugin();
 
     if (!isset($this->cacheKey)) {
       $this->cacheKey = hash('sha256', (string) $source);
@@ -218,7 +218,7 @@
    */
   protected function getIterator() {
     if (!isset($this->iterator)) {
-      $this->iterator = $this->migration->getSource()->getIterator();
+      $this->iterator = $this->migration->getSourcePlugin()->getIterator();
     }
     return $this->iterator;
   }
@@ -261,6 +261,7 @@
     if ($id_list = $this->migration->get('idlist')) {
       $this->idList = $id_list;
     }
+    $this->getIterator()->rewind();
     $this->next();
   }
 
@@ -386,7 +387,7 @@
    */
   protected function prepareRow(Row $row) {
     // We're explicitly skipping this row - keep track in the map table
-    if ($this->migration->getSource()->prepareRow($row) === FALSE) {
+    if ($this->migration->getSourcePlugin()->prepareRow($row) === FALSE) {
       // Make sure we replace any previous messages for this item with any
       // new ones.
       $id_map = $this->migration->getIdMap();
diff -u b/core/modules/migrate/lib/Drupal/migrate/Tests/MigrateSystemConfigsTest.php b/core/modules/migrate/lib/Drupal/migrate/Tests/MigrateSystemConfigsTest.php
--- b/core/modules/migrate/lib/Drupal/migrate/Tests/MigrateSystemConfigsTest.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Tests/MigrateSystemConfigsTest.php
@@ -17,8 +17,8 @@
    */
   public static function getInfo() {
     return array(
-      'name'  => 'Migrate variables to system.site.yml',
-      'description'  => 'Upgrade variables to system.site.yml',
+      'name'  => 'Migrate variables to system.*.yml',
+      'description'  => 'Upgrade variables to system.*.yml',
       'group' => 'Migrate',
     );
   }
@@ -26,7 +26,7 @@
   function testSystemSite() {
     $migration = entity_load('migration', 'd6_system_site');
     $dumps = array(
-      drupal_get_path('module', 'migrate') . '/ib/Drupal/migrate/Tests/Dump/Drupal6SystemSite.php',
+      drupal_get_path('module', 'migrate') . '/lib/Drupal/migrate/Tests/Dump/Drupal6SystemSite.php',
     );
     $this->prepare($migration, $dumps);
     $executable = new MigrateExecutable($migration, new MigrateMessage);
@@ -43,2 +43,18 @@
   }
+
+  /**
+   * Tests migration of book variables to system.cron.yml.
+   */
+  public function testSystemCron() {
+    $migration = entity_load('migration', 'd6_system_cron');
+    $dumps = array(
+      drupal_get_path('module', 'migrate') . '/lib/Drupal/migrate/Tests/Dump/Drupal6SystemCron.php',
+    );
+    $this->prepare($migration, $dumps);
+    $executable = new MigrateExecutable($migration, new MigrateMessage());
+    $executable->import();
+    $config = \Drupal::config('system.cron');
+    $this->assertIdentical($config->get('threshold.warning'), 172800);
+    $this->assertIdentical($config->get('threshold.error'), 1209600);
+  }
 }
diff -u b/core/modules/migrate/lib/Drupal/migrate/Tests/MigrateTestBase.php b/core/modules/migrate/lib/Drupal/migrate/Tests/MigrateTestBase.php
--- b/core/modules/migrate/lib/Drupal/migrate/Tests/MigrateTestBase.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Tests/MigrateTestBase.php
@@ -30,11 +30,15 @@
    * @return \Drupal\Core\Database\Connection
    */
   protected function prepare(MigrationInterface $migration, array $files = array()) {
-    $databasePrefix = 'simpletest_m_' . mt_rand(1000, 1000000);
+    $databasePrefix = 'm_';
     $connection_info = Database::getConnectionInfo('default');
-    $connection_info['default']['prefix']['default'] .= $databasePrefix;
+    foreach ($connection_info as $target => $value) {
+      $connection_info[$target]['prefix'] = array(
+        'default' => $value['prefix']['default'] . $databasePrefix,
+      );
+    }
     $database = SqlBase::getDatabaseConnection($migration->id(), array('database' => $connection_info['default']));
-    foreach (array('source', 'destination', 'id_map') as $key) {
+    foreach (array('source', 'destination', 'idMap') as $key) {
       $configuration = $migration->get($key);
       $configuration['database'] = $database;
       $migration->set($key, $configuration);
@@ -47,7 +51,9 @@
         $file = "compress.zlib://$file";
         require $file;
       }
-      $class = 'Drupal\migrate\Tests\Dump\\' . basename($file, '.php');
+      $namespaces = preg_filter('/^namespace (.*);/', '\1', file($file));
+      // trim() is necessary to remove the line break file() keeps.
+      $class = trim(reset($namespaces)) . '\\' . basename($file, '.php');
       $class::load($database);
     }
     return $database;
diff -u b/core/modules/migrate/migrate.api.php b/core/modules/migrate/migrate.api.php
--- b/core/modules/migrate/migrate.api.php
+++ b/core/modules/migrate/migrate.api.php
@@ -20,6 +20,8 @@
  * For example, filter module used to store filter format settings in the
  * variables table which now needs to be inside the filter format config
  * file. So, it needs to be added here.
+ *
+ * hook_migrate_MIGRATION_ID_prepare_row is also available.
  */
 function hook_migrate_prepare_row(Row $row, MigrateSourceInterface $source, MigrationInterface $migration) {
   if ($migration->id() == 'drupal6_filter_formats') {
diff -u b/core/modules/migrate/tests/Drupal/migrate/Tests/FakeSelect.php b/core/modules/migrate/tests/Drupal/migrate/Tests/FakeSelect.php
--- b/core/modules/migrate/tests/Drupal/migrate/Tests/FakeSelect.php
+++ b/core/modules/migrate/tests/Drupal/migrate/Tests/FakeSelect.php
@@ -8,6 +8,7 @@
 namespace Drupal\migrate\Tests;
 
 use Drupal\Core\Database\Connection;
+use Drupal\Core\Database\DatabaseNotFoundException;
 use Drupal\Core\Database\Query\Condition;
 use Drupal\Core\Database\Query\PlaceholderInterface;
 use Drupal\Core\Database\Query\Select;
@@ -130,7 +131,7 @@
     foreach ($all_rows as $table_rows) {
       $result_row = array();
       foreach ($table_rows as $row) {
-        $result_row += $row;
+        $result_row += $row['result'];
       }
       $results[] = $result_row;
     }
@@ -158,6 +159,18 @@
       $this->fieldsWithTable[$field_info['table'] . '.' . $field_info['field']] = $field_info;
       $fields[$field_info['table']][$field_info['field']] = NULL;
     }
+    foreach ($this->tables as $alias => $table_info) {
+      if ($table = reset($this->databaseContents[$table_info['table']])) {
+        foreach (array_keys($table) as $field) {
+          if (!isset($this->fields[$field])) {
+            $this->fieldsWithTable[$field] = array(
+              'table' => $alias,
+              'field' => $field,
+            );
+          }
+        }
+      }
+    }
 
     $results = array();
     foreach ($this->tables as $table_alias => $table_info) {
@@ -166,13 +179,20 @@
         foreach ($results as $row) {
           $joined = FALSE;
           foreach ($this->databaseContents[$table_info['table']] as $candidate_row) {
-            if ($row[$table_info['original_table_alias']][$table_info['original_field']] == $candidate_row[$table_info['added_field']]) {
+            if ($row[$table_info['original_table_alias']]['result'][$table_info['original_field']] == $candidate_row[$table_info['added_field']]) {
               $joined = TRUE;
               $new_rows[] = $this->getNewRow($table_alias, $fields, $candidate_row, $row);
             }
           }
           if (!$joined && $table_info['join type'] == 'LEFT') {
-            $new_rows[] = array($table_alias => $fields[$table_alias]) + $row;
+            // @TODO: empty tables? Those are a problem.
+            $keys = array_keys($candidate_row);
+            $values = array_fill(0, count($keys), NULL);
+            $new_row = array(
+              'result' => $fields[$table_alias],
+              'all' => array_combine($keys, $values),
+            );
+            $new_rows[] = array($table_alias => $new_row) + $row;
           }
         }
         $results = $new_rows;
@@ -197,9 +217,9 @@
    * @return array
    */
   protected function getNewRow($table_alias, $fields, $candidate_row, $row = array()) {
-    $new_row = array();
+    $new_row[$table_alias]['all'] = $candidate_row;
     foreach ($fields[$table_alias] as $field => $v) {
-      $new_row[$table_alias][$field] = $candidate_row[$field];
+      $new_row[$table_alias]['result'][$field] = $candidate_row[$field];
     }
     return $new_row + $row;
   }
@@ -226,8 +246,8 @@
   protected function sortCallback($a, $b) {
     foreach ($this->order as $field => $direction) {
       $field_info = $this->getFieldInfo($field);
-      $a_value = $a[$field_info['table']][$field_info['field']];
-      $b_value = $b[$field_info['table']][$field_info['field']];
+      $a_value = $this->getValue($a, $field_info);
+      $b_value = $this->getValue($b, $field_info);
       if ($a_value != $b_value) {
         return (($a_value < $b_value) == ($direction == 'ASC')) ? -1 : 1;
       }
@@ -279,6 +299,27 @@
   }
 
   /**
+   * Gets the value of a field from a row.
+   *
+   * @param $row
+   *   The row array, three levels of indexes: first is the table alias, the
+   *   second is either all or result, the third is the field alias.
+   * @param $field_info
+   *   The field information array containing the table alias and the
+   *   field alias.
+   * @return mixed
+   */
+  protected function getValue($row, $field_info) {
+    if (array_key_exists($field_info['field'], $row[$field_info['table']]['result'])) {
+      $index = 'result';
+    }
+    else {
+      $index = 'all';
+    }
+    return $row[$field_info['table']][$index][$field_info['field']];
+  }
+
+  /**
    * Match a single row and its condition.
    *
    * @param array $row
@@ -295,7 +336,7 @@
    */
   protected function matchSingle(array $row, array $condition) {
     $field_info = $this->getFieldInfo($condition['field']);
-    $row_value = $row[$field_info['table']][$field_info['field']];
+    $row_value = $this->getValue($row, $field_info);
     switch ($condition['operator']) {
       case '=':
         return $row_value == $condition['value'];
diff -u b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateExecutableTest.php b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateExecutableTest.php
--- b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateExecutableTest.php
+++ b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateExecutableTest.php
@@ -82,7 +82,7 @@
       ->will($this->returnValue($iterator));
 
     $this->migration->expects($this->any())
-      ->method('getSource')
+      ->method('getSourcePlugin')
       ->will($this->returnValue($source));
 
     // Ensure that a message with the proper message was added.
diff -u b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateSqlSourceTestCase.php b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateSqlSourceTestCase.php
--- b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateSqlSourceTestCase.php
+++ b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateSqlSourceTestCase.php
@@ -48,6 +48,7 @@
     $database->expects($this->any())->method('schema')->will($this->returnCallback(function () use ($database, $database_contents) {
       return new FakeDatabaseSchema($database, $database_contents);
     }));
+    $database->expects($this->any())->method('query')->will($this->throwException(new \Exception('Query is not supported')));
     $module_handler = $this->getMockBuilder('Drupal\Core\Extension\ModuleHandlerInterface')
       ->disableOriginalConstructor()
       ->getMock();
@@ -62,7 +63,7 @@
     $plugin->setDatabase($database);
     $plugin->setModuleHandler($module_handler);
     $migration->expects($this->any())
-      ->method('getSource')
+      ->method('getSourcePlugin')
       ->will($this->returnValue($plugin));
     $migrateExecutable = $this->getMockBuilder('Drupal\migrate\MigrateExecutable')
       ->disableOriginalConstructor()
@@ -102,7 +103,7 @@
   protected function retrievalAssertHelper($expected_value, $actual_value, $message) {
     if (is_array($expected_value)) {
       foreach ($expected_value as $k => $v) {
-        $this->retrievalAssertHelper($v, $actual_value[$k], $message);
+        $this->retrievalAssertHelper($v, $actual_value[$k], $message . '['. $k . ']');
       }
     }
     else {
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate/config/migrate.migration.d6_system_cron.yml
@@ -0,0 +1,13 @@
+id: d6_system_cron
+source:
+    plugin: drupal6_variable
+    variables:
+        - cron_threshold_warning
+        - cron_threshold_error
+        - cron_last
+process:
+    'threshold:warning': cron_threshold_warning
+    'threshold:error': cron_threshold_error
+destination:
+    plugin: d8_config
+    config_name: system.cron
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate/lib/Drupal/migrate/Tests/Dump/Drupal6SystemCron.php
@@ -0,0 +1,59 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate\Tests\Drupal6SystemCron.
+ */
+
+namespace Drupal\migrate\Tests\Dump;
+
+use Drupal\Core\Database\Connection;
+
+/**
+ * Database dump for testing system.cron.yml migration.
+ */
+class Drupal6SystemCron {
+
+  /**
+   * Mock the database schema and values.
+   *
+   * @param \Drupal\Core\Database\Connection $database
+   *   The mocked database connection.
+   */
+  public static function load(Connection $database) {
+    $database->schema()->createTable('variable', array(
+      'fields' => array(
+        'name' => array(
+          'type' => 'varchar',
+          'length' => 128,
+          'not null' => TRUE,
+          'default' => '',
+        ),
+        'value' => array(
+          'type' => 'blob',
+          'not null' => TRUE,
+          'size' => 'big',
+          'translatable' => TRUE,
+        ),
+      ),
+      'primary key' => array(
+        'name',
+      ),
+      'module' => 'system',
+      'name' => 'variable',
+    ));
+    $database->insert('variable')->fields(array(
+      'name',
+      'value',
+    ))
+    ->values(array(
+      'name' => 'cron_threshold_warning',
+      'value' => 'i:172800;',
+    ))
+    ->values(array(
+      'name' => 'cron_threshold_error',
+      'value' => 'i:1209600;',
+    ))
+    ->execute();
+  }
+}