diff --git a/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionAccessTest.php b/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionAccessTest.php
index b7dd8c0..c2a1b78 100644
--- a/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionAccessTest.php
+++ b/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionAccessTest.php
@@ -316,12 +316,8 @@ public function testUserHandler() {
     );
     $this->assertReferenceable($selection_options, $referenceable_tests, 'User handler (admin)');
 
-    // Test the 'include_anonymous' option. This option is intentionally passed
-    // in the backward compatibility 'handler_settings' key to test also that
-    // plugin options passed in this way take precedence over the others, in the
-    // root. In this case the value of FALSE will override the initial value of
-    // 'include_anonymous' even that was declared in root of configuration.
-    $selection_options['handler_settings']['include_anonymous'] = FALSE;
+    // Test the 'include_anonymous' option.
+    $selection_options['include_anonymous'] = FALSE;
     $referenceable_tests = array(
       array(
         'arguments' => array(
diff --git a/core/tests/Drupal/Tests/Core/EntityReferenceSelection/EntityReferenceSelectionUnitTest.php b/core/tests/Drupal/Tests/Core/EntityReferenceSelection/EntityReferenceSelectionUnitTest.php
new file mode 100644
index 0000000..9e14ff9
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/EntityReferenceSelection/EntityReferenceSelectionUnitTest.php
@@ -0,0 +1,110 @@
+<?php
+
+namespace Drupal\Tests\Core\EntityReferenceSelection;
+
+use Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginBase;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Provides unit testing for selection handlers.
+ *
+ * @coversDefaultClass \Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginBase
+ *
+ * @group entity_reference
+ */
+class EntityReferenceSelectionUnitTest extends UnitTestCase {
+
+  /**
+   * Provides test data for ::testSetConfiguration()
+   *
+   * @return array
+   *
+   * @see \Drupal\Tests\Core\EntityReferenceSelection\testSetConfiguration
+   */
+  public function providerTestSetConfiguration() {
+    return [
+      [
+        [
+          'setting1' => 'foo',
+          'setting2' => ['bar', 'baz'],
+        ],
+      ],
+      [
+        [
+          'handler_settings' => [
+            'setting1' => 'foo',
+            'setting2' => ['bar', 'baz'],
+          ],
+        ],
+      ],
+      [
+        [
+          'setting1' => 'foo',
+          'handler_settings' => [
+            'setting2' => ['bar', 'baz'],
+          ]
+        ],
+      ],
+      [
+        [
+          'setting1' => 'foo',
+          'setting2' => 'this will be overwritten',
+          'handler_settings' => [
+            // Settings under 'handler_settings' take precedence.
+            'setting2' => ['bar', 'baz'],
+          ]
+        ],
+      ],
+    ];
+  }
+
+  /**
+   * Tests selection handler plugin configuration set.
+   *
+   * @dataProvider providerTestSetConfiguration
+   * @covers ::setConfiguration
+   *
+   * @param array $options
+   *   The configuration passed to the plugin.
+   */
+  public function testSetConfiguration($options) {
+    $selection = new TestSelection($options, 'test_selector', []);
+    $selection->setConfiguration($options);
+
+    $expected = [
+      'target_type' => NULL,
+      'handler' => 'test_selector',
+      'entity' => NULL,
+      'setting1' => 'foo',
+      'setting2' => [
+        'bar',
+        'baz',
+      ],
+      'handler_settings' => [
+        'setting1' => 'foo',
+        'setting2' => [
+          'bar',
+          'baz',
+        ],
+      ]
+    ];
+
+    $config = $selection->getConfiguration();
+    // Sort by keys to get make comparision possible.
+    $this->assertSame(ksort($expected), ksort($config));
+  }
+
+}
+
+/**
+ * Provides a testing plugin.
+ */
+class TestSelection extends SelectionPluginBase {
+
+  public function getReferenceableEntities($match = NULL, $match_operator = 'CONTAINS', $limit = 0) { }
+
+  public function validateReferenceableEntities(array $ids) { }
+
+  public function countReferenceableEntities($match = NULL, $match_operator = 'CONTAINS') { }
+
+}