only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/filter/tests/src/Unit/Plugin/migrate/process/FilterIdTest.php
@@ -0,0 +1,146 @@
+<?php
+
+namespace Drupal\Tests\filter\Unit\Plugin\migrate\process;
+
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\StringTranslation\TranslationInterface;
+use Drupal\filter\FilterPluginManager;
+use Drupal\filter\Plugin\migrate\process\FilterID;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\Row;
+use Drupal\Tests\UnitTestCase;
+use Prophecy\Argument;
+use Prophecy\Prediction\CallPrediction;
+use Prophecy\Prediction\NoCallsPrediction;
+use Prophecy\Prediction\PredictionInterface;
+
+/**
+ * Unit tests of the filter_id plugin.
+ *
+ * @coversDefaultClass \Drupal\filter\Plugin\migrate\process\FilterID
+ * @group filter
+ */
+class FilterIdTest extends UnitTestCase {
+
+  /**
+   * The FilterPluginManager prophecy.
+   *
+   * @var \Prophecy\Prophecy\ObjectProphecy|FilterPluginManager
+   */
+  protected $filterManager;
+
+  /**
+   * The MigrateExecutable prophecy.
+   *
+   * @var \Prophecy\Prophecy\ObjectProphecy|MigrateExecutableInterface
+   */
+  protected $executable;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->filterManager = $this->prophesize(FilterPluginManager::class);
+    $this->executable = $this->prophesize(MigrateExecutableInterface::class);
+
+    $this->filterManager
+      ->hasDefinition('filter_html')
+      ->willReturn(TRUE);
+
+    $this->filterManager
+      ->hasDefinition('php_code')
+      ->willReturn(FALSE);
+
+    $this->filterManager
+      ->getFallbackPluginId(Argument::type('string'))
+      ->willReturn('filter_null');
+
+    // Mock the string translation service, since the plugin will translate
+    // any message it saves.
+    $translator = $this->prophesize(TranslationInterface::class);
+
+    $translator
+      ->translate(
+        Argument::type('string')
+      )
+      ->willReturnArgument(0);
+
+    $translator
+      ->translate(
+        Argument::type('string'),
+        Argument::type('array')
+      )
+      ->willReturnArgument(0);
+
+    $container = new ContainerBuilder();
+    $container->set('string_translation', $translator->reveal());
+
+    \Drupal::setContainer($container);
+  }
+
+  /**
+   * Tests the filter_id plugin.
+   *
+   * @param mixed $value
+   *   The input value to the plugin.
+   * @param string $expected_value
+   *   The output value expected from the plugin.
+   * @param PredictionInterface $message_saved
+   *   A prediction for if and how the MigrateExecutable's saveMessage() method
+   *   will be called.
+   *
+   * @dataProvider testProvider
+   *
+   * @covers ::transform
+   */
+  public function test($value, $expected_value, PredictionInterface $message_saved) {
+    $plugin = new FilterID(
+      [
+        'bypass' => TRUE,
+        'map' => [
+          'foo' => 'filter_html',
+          'baz' => 'php_code',
+        ],
+      ],
+      'filter_id',
+      [],
+      $this->filterManager->reveal()
+    );
+
+    $this->executable
+      ->saveMessage(
+        Argument::any(),
+        Argument::type('integer')
+      )
+      ->should($message_saved);
+
+    $row = new Row([], []);
+    $output_value = $plugin->transform($value, $this->executable->reveal(), $row, 'foo');
+
+    $this->assertSame($expected_value, $output_value);
+    $this->executable->checkProphecyMethodsPredictions();
+  }
+
+  /**
+   * The test data provider.
+   *
+   * @return array
+   */
+  public function testProvider() {
+    return [
+      // The filter ID is mapped, and the plugin exists.
+      ['foo', 'filter_html', new NoCallsPrediction],
+      // The filter ID isn't mapped, but it's unchanged from the source (i.e.,
+      // it bypasses the static map) and the plugin exists.
+      ['filter_html', 'filter_html', new NoCallsPrediction],
+      // The filter ID is mapped, but the plugin does not exist.
+      ['baz', 'filter_null', new CallPrediction],
+      // The filter ID isn't mapped, but it's unchanged from the source (i.e.,
+      // it bypasses the static map) but the plugin does not exist.
+      ['php_code', 'filter_null', new CallPrediction],
+    ];
+  }
+
+}