Issue #2960056 by Jo Fitzgerald: Add deprecation test to d6/DateField

core/modules/datetime/tests/src/Unit/Plugin/migrate/field/d6/DateFieldTest.php

@@ -24,6 +24,8 @@ class DateFieldTest extends UnitTestCase {
 
   /**
    * Tests an Exception is thrown when the field type is not a known date type.
+   *
+   * @expectedDeprecation DateField is deprecated in Drupal 8.4.x and will be removed before Drupal 9.0.x. Use \Drupal\datetime\Plugin\migrate\field\DateField instead.
    */
   public function testUnknownDateType() {
     $this->migration = $this->prophesize('Drupal\migrate\Plugin\MigrationInterface')->reveal();