Update to Drupal 8.0.0-beta15. For more information, see: https://www.drupal.org/node/2563023

core/modules/aggregator/tests/modules/aggregator_test_views/test_views/views.view.test_aggregator_items.yml

@@ -79,7 +79,12 @@ display:
           table: aggregator_item
           field: timestamp
           id: timestamp
-          plugin_id: date
+          type: timestamp
+          settings:
+            date_format: medium
+            custom_date_format: ''
+            timezone: ''
+          plugin_id: field
           entity_type: aggregator_item
           entity_field: timestamp
         author:

core/modules/aggregator/tests/src/Unit/Plugin/migrate/source/d6/AggregatorFeedTest.php

@@ -20,7 +20,6 @@ class AggregatorFeedTest extends MigrateSqlSourceTestCase {
 
   protected $migrationConfiguration = array(
     'id' => 'test',
-    'idlist' => array(),
     'source' => array(
       'plugin' => 'd6_aggregator_feed',
     ),

core/modules/aggregator/tests/src/Unit/Plugin/migrate/source/d6/AggregatorItemTest.php

@@ -22,8 +22,6 @@ class AggregatorItemTest extends MigrateSqlSourceTestCase {
   protected $migrationConfiguration = array(
     // The ID of the entity, can be any string.
     'id' => 'test',
-    // Leave it empty for now.
-    'idlist' => array(),
     'source' => array(
       'plugin' => 'd6_aggregator_item',
     ),