only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/aggregator/tests/src/Unit/BubbleableMetadataTest.php
@@ -0,0 +1,73 @@
+<?php
+
+namespace Drupal\Tests\aggregator\Unit;
+
+use Drupal\Core\Render\BubbleableMetadata;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Render\BubbleableMetadata
+ * @group aggregator
+ */
+class BubbleableMetadataTest extends UnitTestCase {
+
+  /**
+   * Tests feed asset merging.
+   *
+   * @covers ::mergeAttachments
+   *
+   * @dataProvider providerTestMergeAttachmentsFeedMerging
+   */
+  public function testMergeAttachmentsFeedMerging($a, $b, $expected) {
+    $this->assertSame($expected, BubbleableMetadata::mergeAttachments($a, $b));
+  }
+
+  /**
+   * Data provider for testMergeAttachmentsFeedMerging.
+   *
+   * @return array
+   */
+  public function providerTestMergeAttachmentsFeedMerging() {
+    $feed_a = [
+      'aggregator/rss',
+      'Feed title',
+    ];
+
+    $feed_b = [
+      'taxonomy/term/1/feed',
+      'RSS - foo',
+    ];
+
+    $a = [
+      'feed' => [
+        $feed_a,
+      ],
+    ];
+    $b = [
+      'feed' => [
+        $feed_b,
+      ],
+    ];
+
+    $expected_a = [
+      'feed' => [
+        $feed_a,
+        $feed_b,
+      ],
+    ];
+
+    // Merging in the opposite direction yields the opposite library order.
+    $expected_b = [
+      'feed' => [
+        $feed_b,
+        $feed_a,
+      ],
+    ];
+
+    return [
+      [$a, $b, $expected_a],
+      [$b, $a, $expected_b],
+    ];
+  }
+
+}