Update to Drupal 8.0.0-rc3. For more information, see https://www.drupal.org/node/2608078

core/modules/migrate_drupal/src/Annotation/MigrateCckField.php

@@ -0,0 +1,40 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate_drupal\Annotation\MigrateCckField.
+ */
+
+namespace Drupal\migrate_drupal\Annotation;
+
+use Drupal\Component\Annotation\Plugin;
+
+/**
+ * Defines a cckfield plugin annotation object.
+ *
+ * cckfield plugins are variously responsible for handling the migration of
+ * CCK fields from Drupal 6 to Drupal 8. They are allowed to alter CCK-related
+ * migrations when migrations are being generated, and can compute destination
+ * field types for individual fields during the actual migration process.
+ *
+ * Plugin Namespace: Plugin\migrate\cckfield
+ *
+ * @Annotation
+ */
+class MigrateCckField extends Plugin {
+
+  /**
+   * The plugin ID.
+   *
+   * @var string
+   */
+  public $id;
+
+  /**
+   * Map of D6 and D7 field types to D8 field type plugin IDs.
+   *
+   * @var string[]
+   */
+  public $type_map = [];
+
+}

core/modules/migrate_drupal/src/Plugin/MigrateCckFieldInterface.php

@@ -9,6 +9,7 @@ namespace Drupal\migrate_drupal\Plugin;
 
 use Drupal\Component\Plugin\PluginInspectionInterface;
 use Drupal\migrate\Entity\MigrationInterface;
+use Drupal\migrate\Row;
 
 /**
  * Provides an interface for all CCK field type plugins.
@@ -77,4 +78,15 @@ interface MigrateCckFieldInterface extends PluginInspectionInterface {
    */
   public function processCckFieldValues(MigrationInterface $migration, $field_name, $data);
 
+  /**
+   * Computes the destination type of a migrated field.
+   *
+   * @param \Drupal\migrate\Row $row
+   *   The field being migrated.
+   *
+   * @return string
+   *   The destination field type.
+   */
+  public function getFieldType(Row $row);
+
 }

core/modules/migrate_drupal/src/Plugin/migrate/cckfield/CckFieldPluginBase.php

@@ -9,6 +9,7 @@ namespace Drupal\migrate_drupal\Plugin\migrate\cckfield;
 
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\migrate\Entity\MigrationInterface;
+use Drupal\migrate\Row;
 use Drupal\migrate_drupal\Plugin\MigrateCckFieldInterface;
 
 /**
@@ -70,4 +71,18 @@ abstract class CckFieldPluginBase extends PluginBase implements MigrateCckFieldI
     $migration->mergeProcessOfProperty('options/type', $process);
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getFieldType(Row $row) {
+    $field_type = $row->getSourceProperty('type');
+
+    if (isset($this->pluginDefinition['type_map'][$field_type])) {
+      return $this->pluginDefinition['type_map'][$field_type];
+    }
+    else {
+      return $field_type;
+    }
+  }
+
 }