diff --git a/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentSourceWithHighWaterTest.php b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentSourceWithHighWaterTest.php
new file mode 100644
index 0000000..27fb068
--- /dev/null
+++ b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentSourceWithHighWaterTest.php
@@ -0,0 +1,105 @@
+<?php
+
+namespace Drupal\Tests\comment\Kernel\Plugin\migrate\source\d6;
+
+use Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase;
+
+/**
+ * Tests the Drupal 6 comment source w/ high water handling.
+ *
+ * @covers \Drupal\comment\Plugin\migrate\source\d6\Comment
+ *
+ * @group comment
+ */
+class CommentSourceWithHighWaterTest extends MigrateSqlSourceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['comment', 'migrate_drupal'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public function providerSource() {
+    $tests = [];
+
+    // The source data.
+    $tests[0]['source_data']['comments'] = [
+      [
+        'cid' => 1,
+        'pid' => 0,
+        'nid' => 2,
+        'uid' => 3,
+        'subject' => 'subject value 1',
+        'comment' => 'comment value 1',
+        'hostname' => 'hostname value 1',
+        'timestamp' => 1382255613,
+        'status' => 0,
+        'thread' => '',
+        'name' => '',
+        'mail' => '',
+        'homepage' => '',
+        'format' => 'testformat1',
+        'type' => 'story',
+      ],
+      [
+        'cid' => 2,
+        'pid' => 1,
+        'nid' => 3,
+        'uid' => 4,
+        'subject' => 'subject value 2',
+        'comment' => 'comment value 2',
+        'hostname' => 'hostname value 2',
+        'timestamp' => 1382255662,
+        'status' => 0,
+        'thread' => '',
+        'name' => '',
+        'mail' => '',
+        'homepage' => '',
+        'format' => 'testformat2',
+        'type' => 'page',
+      ],
+    ];
+
+    $tests[0]['source_data']['node'] = [
+      [
+        'nid' => 2,
+        'type' => 'story',
+      ],
+      [
+        'nid' => 3,
+        'type' => 'page',
+      ],
+    ];
+
+    // The expected results.
+    $tests[0]['expected_data'] = [
+      [
+        'cid' => 2,
+        'pid' => 1,
+        'nid' => 3,
+        'uid' => 4,
+        'subject' => 'subject value 2',
+        'comment' => 'comment value 2',
+        'hostname' => 'hostname value 2',
+        'timestamp' => 1382255662,
+        'status' => 1,
+        'thread' => '',
+        'name' => '',
+        'mail' => '',
+        'homepage' => '',
+        'format' => 'testformat2',
+        'type' => 'page',
+      ],
+    ];
+
+    // The expected count.
+    $tests[0]['expected_count'] = NULL;
+
+    $tests[0]['configuration']['high_water_property']['name'] = 'timestamp';
+    $tests[0]['high_water'] = $tests[0]['source_data']['comments'][0]['timestamp'];
+    return $tests;
+  }
+
+}
diff --git a/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentTest.php b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentTest.php
new file mode 100644
index 0000000..0372f0f
--- /dev/null
+++ b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentTest.php
@@ -0,0 +1,116 @@
+<?php
+
+namespace Drupal\Tests\comment\Kernel\Plugin\migrate\source\d6;
+
+use Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase;
+
+/**
+ * Tests D6 comment source plugin.
+ *
+ * @covers \Drupal\comment\Plugin\migrate\source\d6\Comment
+ * @group comment
+ */
+class CommentTest extends MigrateSqlSourceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['comment', 'migrate_drupal'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public function providerSource() {
+    $tests = [];
+
+    // The source data.
+    $tests[0]['source_data']['comments'] = [
+      [
+        'cid' => 1,
+        'pid' => 0,
+        'nid' => 2,
+        'uid' => 3,
+        'subject' => 'subject value 1',
+        'comment' => 'comment value 1',
+        'hostname' => 'hostname value 1',
+        'timestamp' => 1382255613,
+        'status' => 0,
+        'thread' => '',
+        'name' => '',
+        'mail' => '',
+        'homepage' => '',
+        'format' => 'testformat1',
+        'type' => 'story',
+      ],
+      [
+        'cid' => 2,
+        'pid' => 1,
+        'nid' => 3,
+        'uid' => 4,
+        'subject' => 'subject value 2',
+        'comment' => 'comment value 2',
+        'hostname' => 'hostname value 2',
+        'timestamp' => 1382255662,
+        'status' => 0,
+        'thread' => '',
+        'name' => '',
+        'mail' => '',
+        'homepage' => '',
+        'format' => 'testformat2',
+        'type' => 'page',
+      ],
+    ];
+
+    $tests[0]['source_data']['node'] = [
+      [
+        'nid' => 2,
+        'type' => 'story',
+      ],
+      [
+        'nid' => 3,
+        'type' => 'page',
+      ],
+    ];
+
+    // The expected results.
+    $tests[0]['expected_data'] = [
+      [
+        'cid' => 1,
+        'pid' => 0,
+        'nid' => 2,
+        'uid' => 3,
+        'subject' => 'subject value 1',
+        'comment' => 'comment value 1',
+        'hostname' => 'hostname value 1',
+        'timestamp' => 1382255613,
+        'status' => 1,
+        'thread' => '',
+        'name' => '',
+        'mail' => '',
+        'homepage' => '',
+        'format' => 'testformat1',
+        'type' => 'story',
+      ],
+      [
+        'cid' => 2,
+        'pid' => 1,
+        'nid' => 3,
+        'uid' => 4,
+        'subject' => 'subject value 2',
+        'comment' => 'comment value 2',
+        'hostname' => 'hostname value 2',
+        'timestamp' => 1382255662,
+        'status' => 1,
+        'thread' => '',
+        'name' => '',
+        'mail' => '',
+        'homepage' => '',
+        'format' => 'testformat2',
+        'type' => 'page',
+      ],
+    ];
+
+    return $tests;
+  }
+
+}
diff --git a/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentVariablePerCommentTypeTest.php b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentVariablePerCommentTypeTest.php
new file mode 100644
index 0000000..dd1b1fd
--- /dev/null
+++ b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentVariablePerCommentTypeTest.php
@@ -0,0 +1,62 @@
+<?php
+
+namespace Drupal\Tests\comment\Kernel\Plugin\migrate\source\d6;
+
+use Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase;
+
+/**
+ * Tests comment variable per comment type source plugin.
+ *
+ * @covers \Drupal\comment\Plugin\migrate\source\d6\CommentVariablePerCommentType
+ * @group comment
+ */
+class CommentVariablePerCommentTypeTest extends MigrateSqlSourceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['comment', 'migrate_drupal'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public function providerSource() {
+    $tests = [];
+
+    // The source data.
+    $tests[0]['source_data']['node_type'] = [
+      [
+        'type' => 'page',
+      ],
+      [
+        'type' => 'story',
+      ],
+    ];
+
+    $tests[0]['source_data']['variable'] = [
+      [
+        'name' => 'comment_subject_field_page',
+        'value' => serialize(1),
+      ],
+      [
+        'name' => 'comment_subject_field_story',
+        'value' => serialize(0),
+      ],
+    ];
+
+    // The expected results.
+    // Each result will also include a label and description, but those are
+    // static values set by the source plugin and don't need to be asserted.
+    $tests[0]['expected_data'] = [
+      [
+        'comment_type' => 'comment',
+      ],
+      [
+        'comment_type' => 'comment_no_subject',
+      ],
+    ];
+
+    return $tests;
+  }
+
+}
diff --git a/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentVariableTest.php b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentVariableTest.php
new file mode 100644
index 0000000..a84c776
--- /dev/null
+++ b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentVariableTest.php
@@ -0,0 +1,92 @@
+<?php
+
+namespace Drupal\Tests\comment\Kernel\Plugin\migrate\source\d6;
+
+use Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase;
+
+/**
+ * Tests d6_comment_variable source plugin.
+ *
+ * @covers \Drupal\comment\Plugin\migrate\source\d6\CommentVariable
+ * @group comment
+ */
+class CommentVariableTest extends MigrateSqlSourceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['comment', 'migrate_drupal'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public function providerSource() {
+    $tests = [];
+
+    // The source data.
+    $tests[0]['source_data']['node_type'] = [
+      [
+        'type' => 'page',
+      ],
+    ];
+
+    $tests[0]['source_data']['variable'] = [
+      [
+        'name' => 'comment_page',
+        'value' => serialize(1),
+      ],
+      [
+        'name' => 'comment_default_mode_page',
+        'value' => serialize(1),
+      ],
+      [
+        'name' => 'comment_default_order_page',
+        'value' => serialize(1),
+      ],
+      [
+        'name' => 'comment_default_per_page_page',
+        'value' => serialize(50),
+      ],
+      [
+        'name' => 'comment_controls_page',
+        'value' => serialize(1),
+      ],
+      [
+        'name' => 'comment_anonymous_page',
+        'value' => serialize(1),
+      ],
+      [
+        'name' => 'comment_subject_field_page',
+        'value' => serialize(1),
+      ],
+      [
+        'name' => 'comment_preview_page',
+        'value' => serialize(1),
+      ],
+      [
+        'name' => 'comment_form_location_page',
+        'value' => serialize(1),
+      ],
+    ];
+
+    // The expected results.
+    $tests[0]['expected_data'] = [
+      [
+        'comment' => '1',
+        'comment_default_mode' => '1',
+        'comment_default_order' => '1',
+        'comment_default_per_page' => '50',
+        'comment_controls' => '1',
+        'comment_anonymous' => '1',
+        'comment_subject_field' => '1',
+        'comment_preview' => '1',
+        'comment_form_location' => '1',
+        'node_type' => 'page',
+        'comment_type' => 'comment',
+      ],
+    ];
+
+    return $tests;
+  }
+
+}
diff --git a/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d7/CommentTest.php b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d7/CommentTest.php
new file mode 100644
index 0000000..9d9760b
--- /dev/null
+++ b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d7/CommentTest.php
@@ -0,0 +1,117 @@
+<?php
+
+namespace Drupal\Tests\comment\Kernel\Plugin\migrate\source\d7;
+
+use Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase;
+
+/**
+ * Tests D7 comment source plugin.
+ *
+ * @covers \Drupal\comment\Plugin\migrate\source\d7\Comment
+ * @group comment
+ */
+class CommentTest extends MigrateSqlSourceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['comment', 'migrate_drupal'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public function providerSource() {
+    $tests = [];
+
+    // The source data.
+    $tests[0]['source_data']['comment'] = [
+      [
+        'cid' => '1',
+        'pid' => '0',
+        'nid' => '1',
+        'uid' => '1',
+        'subject' => 'A comment',
+        'hostname' => '::1',
+        'created' => '1421727536',
+        'changed' => '1421727536',
+        'status' => '1',
+        'thread' => '01/',
+        'name' => 'admin',
+        'mail' => '',
+        'homepage' => '',
+        'language' => 'und',
+      ],
+    ];
+    $tests[0]['source_data']['node'] = [
+      [
+        'nid' => '1',
+        'vid' => '1',
+        'type' => 'test_content_type',
+        'language' => 'en',
+        'title' => 'A Node',
+        'uid' => '1',
+        'status' => '1',
+        'created' => '1421727515',
+        'changed' => '1421727515',
+        'comment' => '2',
+        'promote' => '1',
+        'sticky' => '0',
+        'tnid' => '0',
+        'translate' => '0',
+      ],
+    ];
+    $tests[0]['source_data']['field_config_instance'] = [
+      [
+        'id' => '14',
+        'field_id' => '1',
+        'field_name' => 'comment_body',
+        'entity_type' => 'comment',
+        'bundle' => 'comment_node_test_content_type',
+        'data' => 'a:0:{}',
+        'deleted' => '0',
+      ],
+    ];
+    $tests[0]['source_data']['field_data_comment_body'] = [
+      [
+        'entity_type' => 'comment',
+        'bundle' => 'comment_node_test_content_type',
+        'deleted' => '0',
+        'entity_id' => '1',
+        'revision_id' => '1',
+        'language' => 'und',
+        'delta' => '0',
+        'comment_body_value' => 'This is a comment',
+        'comment_body_format' => 'filtered_html',
+      ],
+    ];
+
+    // The expected results.
+    $tests[0]['expected_data'] = [
+      [
+        'cid' => '1',
+        'pid' => '0',
+        'nid' => '1',
+        'uid' => '1',
+        'subject' => 'A comment',
+        'hostname' => '::1',
+        'created' => '1421727536',
+        'changed' => '1421727536',
+        'status' => '1',
+        'thread' => '01/',
+        'name' => 'admin',
+        'mail' => '',
+        'homepage' => '',
+        'language' => 'und',
+        'comment_body' => [
+          [
+            'value' => 'This is a comment',
+            'format' => 'filtered_html',
+          ],
+        ],
+      ],
+    ];
+
+    return $tests;
+  }
+
+}
diff --git a/core/modules/comment/tests/src/Unit/Migrate/d7/CommentTypeTest.php b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d7/CommentTypeTest.php
similarity index 60%
rename from core/modules/comment/tests/src/Unit/Migrate/d7/CommentTypeTest.php
rename to core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d7/CommentTypeTest.php
index 106716d..11f3db5 100644
--- a/core/modules/comment/tests/src/Unit/Migrate/d7/CommentTypeTest.php
+++ b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d7/CommentTypeTest.php
@@ -1,44 +1,30 @@
 <?php
 
-namespace Drupal\Tests\comment\Unit\Migrate\d7;
+namespace Drupal\Tests\comment\Kernel\Plugin\migrate\source\d7;
 
-use Drupal\Tests\migrate\Unit\MigrateSqlSourceTestCase;
+use Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase;
 
 /**
  * Tests D7 comment type source plugin.
  *
+ * @covers \Drupal\comment\Plugin\migrate\source\d7\CommentType
  * @group comment
  */
-class CommentTypeTest extends MigrateSqlSourceTestCase {
+class CommentTypeTest extends MigrateSqlSourceTestBase {
 
-  const PLUGIN_CLASS = 'Drupal\comment\Plugin\migrate\source\d7\CommentType';
-
-  protected $migrationConfiguration = [
-    'id' => 'test',
-    'source' => [
-      'plugin' => 'd7_comment_type',
-    ],
-  ];
-
-  protected $expectedResults = [
-    [
-      'bundle' => 'comment_node_article',
-      'node_type' => 'article',
-      'default_mode' => '1',
-      'per_page' => '50',
-      'anonymous' => '0',
-      'form_location' => '1',
-      'preview' => '0',
-      'subject' => '1',
-      'label' => 'Article comment',
-    ],
-  ];
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['comment', 'migrate_drupal'];
 
   /**
    * {@inheritdoc}
    */
-  protected function setUp() {
-    $this->databaseContents['node_type'] = [
+  public function providerSource() {
+    $tests = [];
+
+    // The source data.
+    $tests[0]['source_data']['node_type'] = [
       [
         'type' => 'article',
         'name' => 'Article',
@@ -55,7 +41,7 @@ protected function setUp() {
         'orig_type' => 'article',
       ],
     ];
-    $this->databaseContents['field_config_instance'] = [
+    $tests[0]['source_data']['field_config_instance'] = [
       [
         'id' => '14',
         'field_id' => '1',
@@ -66,7 +52,7 @@ protected function setUp() {
         'deleted' => '0',
       ],
     ];
-    $this->databaseContents['variable'] = [
+    $tests[0]['source_data']['variable'] = [
       [
         'name' => 'comment_default_mode_article',
         'value' => serialize(1),
@@ -92,7 +78,22 @@ protected function setUp() {
         'value' => serialize(1),
       ],
     ];
-    parent::setUp();
+
+    // The expected results.
+    $tests[0]['expected_data'] = [
+      [
+        'bundle' => 'comment_node_article',
+        'node_type' => 'article',
+        'default_mode' => '1',
+        'per_page' => '50',
+        'anonymous' => '0',
+        'form_location' => '1',
+        'preview' => '0',
+        'subject' => '1',
+        'label' => 'Article comment',
+      ],
+    ];
+    return $tests;
   }
 
 }
diff --git a/core/modules/comment/tests/src/Unit/Migrate/d6/CommentSourceWithHighWaterTest.php b/core/modules/comment/tests/src/Unit/Migrate/d6/CommentSourceWithHighWaterTest.php
deleted file mode 100644
index b2cbaa7..0000000
--- a/core/modules/comment/tests/src/Unit/Migrate/d6/CommentSourceWithHighWaterTest.php
+++ /dev/null
@@ -1,23 +0,0 @@
-<?php
-
-namespace Drupal\Tests\comment\Unit\Migrate\d6;
-
-/**
- * Tests the Drupal 6 comment source w/ high water handling.
- *
- * @group comment
- */
-class CommentSourceWithHighWaterTest extends CommentTestBase {
-
-  const ORIGINAL_HIGH_WATER = 1382255613;
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function setUp() {
-    $this->migrationConfiguration['source']['high_water_property']['name'] = 'timestamp';
-    array_shift($this->expectedResults);
-    parent::setUp();
-  }
-
-}
diff --git a/core/modules/comment/tests/src/Unit/Migrate/d6/CommentTest.php b/core/modules/comment/tests/src/Unit/Migrate/d6/CommentTest.php
deleted file mode 100644
index 2056d24..0000000
--- a/core/modules/comment/tests/src/Unit/Migrate/d6/CommentTest.php
+++ /dev/null
@@ -1,12 +0,0 @@
-<?php
-
-namespace Drupal\Tests\comment\Unit\Migrate\d6;
-
-/**
- * Tests D6 comment source plugin.
- *
- * @group comment
- */
-class CommentTest extends CommentTestBase {
-
-}
diff --git a/core/modules/comment/tests/src/Unit/Migrate/d6/CommentTestBase.php b/core/modules/comment/tests/src/Unit/Migrate/d6/CommentTestBase.php
deleted file mode 100644
index 220328b..0000000
--- a/core/modules/comment/tests/src/Unit/Migrate/d6/CommentTestBase.php
+++ /dev/null
@@ -1,80 +0,0 @@
-<?php
-
-namespace Drupal\Tests\comment\Unit\Migrate\d6;
-
-use Drupal\Tests\migrate\Unit\MigrateSqlSourceTestCase;
-
-/**
- * Base class for comment source unit tests.
- */
-abstract class CommentTestBase extends MigrateSqlSourceTestCase {
-
-  // The plugin system is not working during unit testing so the source plugin
-  // class needs to be manually specified.
-  const PLUGIN_CLASS = 'Drupal\comment\Plugin\migrate\source\d6\Comment';
-
-  // The fake Migration configuration entity.
-  protected $migrationConfiguration = [
-    // The ID of the entity, can be any string.
-    'id' => 'test',
-    // This needs to be the identifier of the actual key: cid for comment, nid
-    // for node and so on.
-    'source' => [
-      'plugin' => 'd6_comment',
-    ],
-  ];
-
-  // We need to set up the database contents; it's easier to do that below.
-
-  protected $expectedResults = [
-    [
-      'cid' => 1,
-      'pid' => 0,
-      'nid' => 2,
-      'uid' => 3,
-      'subject' => 'subject value 1',
-      'comment' => 'comment value 1',
-      'hostname' => 'hostname value 1',
-      'timestamp' => 1382255613,
-      'status' => 1,
-      'thread' => '',
-      'name' => '',
-      'mail' => '',
-      'homepage' => '',
-      'format' => 'testformat1',
-      'type' => 'story',
-    ],
-    [
-      'cid' => 2,
-      'pid' => 1,
-      'nid' => 3,
-      'uid' => 4,
-      'subject' => 'subject value 2',
-      'comment' => 'comment value 2',
-      'hostname' => 'hostname value 2',
-      'timestamp' => 1382255662,
-      'status' => 1,
-      'thread' => '',
-      'name' => '',
-      'mail' => '',
-      'homepage' => '',
-      'format' => 'testformat2',
-      'type' => 'page',
-    ],
-  ];
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function setUp() {
-    foreach ($this->expectedResults as $k => $row) {
-      $this->databaseContents['comments'][$k] = $row;
-      $this->databaseContents['comments'][$k]['status'] = 1 - $this->databaseContents['comments'][$k]['status'];
-    }
-    // Add node table data.
-    $this->databaseContents['node'][] = ['nid' => 2, 'type' => 'story'];
-    $this->databaseContents['node'][] = ['nid' => 3, 'type' => 'page'];
-    parent::setUp();
-  }
-
-}
diff --git a/core/modules/comment/tests/src/Unit/Migrate/d6/CommentVariablePerCommentTypeTest.php b/core/modules/comment/tests/src/Unit/Migrate/d6/CommentVariablePerCommentTypeTest.php
deleted file mode 100644
index 892319a..0000000
--- a/core/modules/comment/tests/src/Unit/Migrate/d6/CommentVariablePerCommentTypeTest.php
+++ /dev/null
@@ -1,59 +0,0 @@
-<?php
-
-namespace Drupal\Tests\comment\Unit\Migrate\d6;
-
-use Drupal\comment\Plugin\migrate\source\d6\CommentVariablePerCommentType;
-use Drupal\Tests\migrate\Unit\MigrateSqlSourceTestCase;
-
-/**
- * @coversDefaultClass \Drupal\comment\Plugin\migrate\source\d6\CommentVariablePerCommentType
- * @group comment
- */
-class CommentVariablePerCommentTypeTest extends MigrateSqlSourceTestCase {
-
-  const PLUGIN_CLASS = CommentVariablePerCommentType::class;
-
-  protected $migrationConfiguration = [
-    'id' => 'test',
-    'source' => [
-      'plugin' => 'd6_comment_variable_per_comment_type',
-    ],
-  ];
-
-  protected $expectedResults = [
-    // Each result will also include a label and description, but those are
-    // static values set by the source plugin and don't need to be asserted.
-    [
-      'comment_type' => 'comment',
-    ],
-    [
-      'comment_type' => 'comment_no_subject',
-    ],
-  ];
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function setUp() {
-    $this->databaseContents['node_type'] = [
-      [
-        'type' => 'page',
-      ],
-      [
-        'type' => 'story',
-      ],
-    ];
-    $this->databaseContents['variable'] = [
-      [
-        'name' => 'comment_subject_field_page',
-        'value' => serialize(1),
-      ],
-      [
-        'name' => 'comment_subject_field_story',
-        'value' => serialize(0),
-      ],
-    ];
-    parent::setUp();
-  }
-
-}
diff --git a/core/modules/comment/tests/src/Unit/Migrate/d6/CommentVariableTest.php b/core/modules/comment/tests/src/Unit/Migrate/d6/CommentVariableTest.php
deleted file mode 100644
index 406cd11..0000000
--- a/core/modules/comment/tests/src/Unit/Migrate/d6/CommentVariableTest.php
+++ /dev/null
@@ -1,89 +0,0 @@
-<?php
-
-namespace Drupal\Tests\comment\Unit\Migrate\d6;
-
-use Drupal\comment\Plugin\migrate\source\d6\CommentVariable;
-use Drupal\Tests\migrate\Unit\MigrateSqlSourceTestCase;
-
-/**
- * @coversDefaultClass \Drupal\comment\Plugin\migrate\source\d6\CommentVariable
- * @group comment
- */
-class CommentVariableTest extends MigrateSqlSourceTestCase {
-
-  const PLUGIN_CLASS = CommentVariable::class;
-
-  protected $migrationConfiguration = [
-    'id' => 'test',
-    'source' => [
-      'plugin' => 'd6_comment_variable',
-    ],
-  ];
-
-  protected $expectedResults = [
-    [
-      'comment' => '1',
-      'comment_default_mode' => '1',
-      'comment_default_order' => '1',
-      'comment_default_per_page' => '50',
-      'comment_controls' => '1',
-      'comment_anonymous' => '1',
-      'comment_subject_field' => '1',
-      'comment_preview' => '1',
-      'comment_form_location' => '1',
-      'node_type' => 'page',
-      'comment_type' => 'comment',
-    ],
-  ];
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function setUp() {
-    $this->databaseContents['node_type'] = [
-      [
-        'type' => 'page',
-      ],
-    ];
-    $this->databaseContents['variable'] = [
-      [
-        'name' => 'comment_page',
-        'value' => serialize(1),
-      ],
-      [
-        'name' => 'comment_default_mode_page',
-        'value' => serialize(1),
-      ],
-      [
-        'name' => 'comment_default_order_page',
-        'value' => serialize(1),
-      ],
-      [
-        'name' => 'comment_default_per_page_page',
-        'value' => serialize(50),
-      ],
-      [
-        'name' => 'comment_controls_page',
-        'value' => serialize(1),
-      ],
-      [
-        'name' => 'comment_anonymous_page',
-        'value' => serialize(1),
-      ],
-      [
-        'name' => 'comment_subject_field_page',
-        'value' => serialize(1),
-      ],
-      [
-        'name' => 'comment_preview_page',
-        'value' => serialize(1),
-      ],
-      [
-        'name' => 'comment_form_location_page',
-        'value' => serialize(1),
-      ],
-    ];
-    parent::setUp();
-  }
-
-}
diff --git a/core/modules/comment/tests/src/Unit/Migrate/d7/CommentTest.php b/core/modules/comment/tests/src/Unit/Migrate/d7/CommentTest.php
deleted file mode 100644
index aed2f28..0000000
--- a/core/modules/comment/tests/src/Unit/Migrate/d7/CommentTest.php
+++ /dev/null
@@ -1,100 +0,0 @@
-<?php
-
-namespace Drupal\Tests\comment\Unit\Migrate\d7;
-
-use Drupal\Tests\migrate\Unit\MigrateSqlSourceTestCase;
-
-/**
- * Tests D7 comment source plugin.
- *
- * @group comment
- */
-class CommentTest extends MigrateSqlSourceTestCase {
-
-  const PLUGIN_CLASS = 'Drupal\comment\Plugin\migrate\source\d7\Comment';
-
-  protected $migrationConfiguration = [
-    'id' => 'test',
-    'source' => [
-      'plugin' => 'd7_comment',
-    ],
-  ];
-
-  protected $expectedResults = [
-    [
-      'cid' => '1',
-      'pid' => '0',
-      'nid' => '1',
-      'uid' => '1',
-      'subject' => 'A comment',
-      'hostname' => '::1',
-      'created' => '1421727536',
-      'changed' => '1421727536',
-      'status' => '1',
-      'thread' => '01/',
-      'name' => 'admin',
-      'mail' => '',
-      'homepage' => '',
-      'language' => 'und',
-      'comment_body' => [
-        [
-          'value' => 'This is a comment',
-          'format' => 'filtered_html',
-        ],
-      ],
-    ],
-  ];
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function setUp() {
-    $this->databaseContents['comment'] = $this->expectedResults;
-    unset($this->databaseContents['comment'][0]['comment_body']);
-
-    $this->databaseContents['node'] = [
-      [
-        'nid' => '1',
-        'vid' => '1',
-        'type' => 'test_content_type',
-        'language' => 'en',
-        'title' => 'A Node',
-        'uid' => '1',
-        'status' => '1',
-        'created' => '1421727515',
-        'changed' => '1421727515',
-        'comment' => '2',
-        'promote' => '1',
-        'sticky' => '0',
-        'tnid' => '0',
-        'translate' => '0',
-      ],
-    ];
-    $this->databaseContents['field_config_instance'] = [
-      [
-        'id' => '14',
-        'field_id' => '1',
-        'field_name' => 'comment_body',
-        'entity_type' => 'comment',
-        'bundle' => 'comment_node_test_content_type',
-        'data' => 'a:0:{}',
-        'deleted' => '0',
-      ],
-    ];
-    $this->databaseContents['field_data_comment_body'] = [
-      [
-        'entity_type' => 'comment',
-        'bundle' => 'comment_node_test_content_type',
-        'deleted' => '0',
-        'entity_id' => '1',
-        'revision_id' => '1',
-        'language' => 'und',
-        'delta' => '0',
-        'comment_body_value' => 'This is a comment',
-        'comment_body_format' => 'filtered_html',
-      ],
-    ];
-    parent::setUp();
-  }
-
-}
diff --git a/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
index 380814c..a8c66b2 100644
--- a/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
@@ -274,6 +274,7 @@ protected function initializeIterator() {
         $high_water_field = $this->getHighWaterField();
         $conditions->condition($high_water_field, $high_water, '>');
         $this->query->orderBy($high_water_field);
+        $condition_added = TRUE;
       }
       if ($condition_added) {
         $this->query->condition($conditions);
diff --git a/core/modules/migrate/tests/src/Kernel/MigrateSourceTestBase.php b/core/modules/migrate/tests/src/Kernel/MigrateSourceTestBase.php
index e3b2f8c..7943e7c 100644
--- a/core/modules/migrate/tests/src/Kernel/MigrateSourceTestBase.php
+++ b/core/modules/migrate/tests/src/Kernel/MigrateSourceTestBase.php
@@ -135,21 +135,31 @@ protected function getPlugin(array $configuration) {
    *   value (like FALSE or 'nope'), the source plugin will not be counted.
    * @param array $configuration
    *   (optional) Configuration for the source plugin.
+   * @param mixed $high_water
+   *   (optional) The value of the high water field.
    *
    * @dataProvider providerSource
    */
-  public function testSource(array $source_data, array $expected_data, $expected_count = NULL, array $configuration = []) {
+  public function testSource(array $source_data, array $expected_data, $expected_count = NULL, array $configuration = [], $high_water = NULL) {
     $plugin = $this->getPlugin($configuration);
 
     // All source plugins must define IDs.
     $this->assertNotEmpty($plugin->getIds());
 
+    // If there is a high water mark, set it in the high water storage.
+    if (isset($high_water)) {
+      $this->container
+        ->get('keyvalue')
+        ->get('migrate:high_water')
+        ->set($this->migration->reveal()->id(), $high_water);
+    }
+
     if (is_null($expected_count)) {
       $expected_count = count($expected_data);
     }
     // If an expected count was given, assert it only if the plugin is
-    // countable.
-    if (is_numeric($expected_count)) {
+    // countable and there is no high water mark.
+    if (is_numeric($expected_count) && !isset($high_water)) {
       $this->assertInstanceOf('\Countable', $plugin);
       $this->assertCount($expected_count, $plugin);
     }
@@ -175,6 +185,10 @@ public function testSource(array $source_data, array $expected_data, $expected_c
         }
       }
     }
+
+    if (isset($high_water)) {
+      $this->assertSame($expected_count, $i);
+    }
   }
 
 }
diff --git a/core/modules/migrate/tests/src/Kernel/MigrateSqlSourceTestBase.php b/core/modules/migrate/tests/src/Kernel/MigrateSqlSourceTestBase.php
index f04183f..90187c9 100644
--- a/core/modules/migrate/tests/src/Kernel/MigrateSqlSourceTestBase.php
+++ b/core/modules/migrate/tests/src/Kernel/MigrateSqlSourceTestBase.php
@@ -63,12 +63,14 @@ protected function getDatabase(array $source_data) {
    *   (optional) How many rows the source plugin is expected to return.
    * @param array $configuration
    *   (optional) Configuration for the source plugin.
+   * @param mixed $high_water
+   *   (optional) The value of the high water field.
    *
    * @dataProvider providerSource
    *
    * @requires extension pdo_sqlite
    */
-  public function testSource(array $source_data, array $expected_data, $expected_count = NULL, array $configuration = []) {
+  public function testSource(array $source_data, array $expected_data, $expected_count = NULL, array $configuration = [], $high_water = NULL) {
     $plugin = $this->getPlugin($configuration);
 
     // Since we don't yet inject the database connection, we need to use a
@@ -78,7 +80,7 @@ public function testSource(array $source_data, array $expected_data, $expected_c
     $property->setAccessible(TRUE);
     $property->setValue($plugin, $this->getDatabase($source_data));
 
-    parent::testSource($source_data, $expected_data, $expected_count, $configuration);
+    parent::testSource($source_data, $expected_data, $expected_count, $configuration, $high_water);
   }
 
 }