MC-5596: Comments are not being added in Credit memo emails

Author: Joan He

app/code/Magento/Sales/Model/Order/Creditmemo.php

@@ -19,10 +19,10 @@
  * Order creditmemo model
  *
  * @api
- * @method \Magento\Sales\Model\Order\Invoice setSendEmail(bool $value)
- * @method \Magento\Sales\Model\Order\Invoice setCustomerNote(string $value)
+ * @method \Magento\Sales\Model\Order\Creditmemo setSendEmail(bool $value)
+ * @method \Magento\Sales\Model\Order\Creditmemo setCustomerNote(string $value)
  * @method string getCustomerNote()
- * @method \Magento\Sales\Model\Order\Invoice setCustomerNoteNotify(bool $value)
+ * @method \Magento\Sales\Model\Order\Creditmemo setCustomerNoteNotify(bool $value)
  * @method bool getCustomerNoteNotify()
  * @SuppressWarnings(PHPMD.ExcessivePublicCount)
  * @SuppressWarnings(PHPMD.ExcessiveClassComplexity)

app/code/Magento/Sales/Model/Order/CreditmemoDocumentFactory.php

@@ -99,11 +99,13 @@ private function attachComment(
             ->setCreditmemo($creditmemo)
             ->setIsCustomerNotified($appendComment);
         $creditmemo->setComments([$comment]);
+        $creditmemo->setCustomerNote($comment->getComment());
         return $creditmemo;
     }
 
     /**
      * Create new Creditmemo
+     *
      * @param \Magento\Sales\Api\Data\OrderInterface $order
      * @param \Magento\Sales\Api\Data\CreditmemoItemCreationInterface[] $items
      * @param \Magento\Sales\Api\Data\CreditmemoCommentCreationInterface|null $comment
@@ -128,6 +130,8 @@ public function createFromOrder(
     }
 
     /**
+     * Create credit memo from invoice
+     *
      * @param \Magento\Sales\Api\Data\InvoiceInterface $invoice
      * @param \Magento\Sales\Api\Data\CreditmemoItemCreationInterface[] $items
      * @param \Magento\Sales\Api\Data\CreditmemoCommentCreationInterface|null $comment

app/code/Magento/Sales/Test/Unit/Model/Order/CreditmemoDocumentFactoryTest.php

@@ -12,7 +12,6 @@
 use Magento\Sales\Api\Data\CreditmemoCommentInterface;
 use Magento\Sales\Model\Order;
 use Magento\Sales\Model\Order\Invoice;
-use Magento\Sales\Api\Data\CreditmemoInterface;
 use Magento\Sales\Api\Data\CreditmemoItemCreationInterface;
 use Magento\Sales\Api\Data\CreditmemoCommentCreationInterface;
 use Magento\Framework\EntityManager\HydratorPool;
@@ -82,7 +81,7 @@ class CreditmemoDocumentFactoryTest extends \PHPUnit\Framework\TestCase
     private $commentCreationArgumentsMock;
 
     /**
-     * @var CreditmemoInterface|\PHPUnit_Framework_MockObject_MockObject
+     * @var Order\Creditmemo|\PHPUnit_Framework_MockObject_MockObject
      */
     private $creditmemoMock;
 
@@ -121,7 +120,7 @@ public function setUp()
         $this->creditmemoItemCreationMock = $this->getMockBuilder(CreditmemoItemCreationInterface::class)
             ->disableOriginalConstructor()
             ->getMock();
-        $this->creditmemoMock = $this->getMockBuilder(CreditmemoInterface::class)
+        $this->creditmemoMock = $this->getMockBuilder(Order\Creditmemo::class)
             ->disableOriginalConstructor()
             ->getMock();
         $this->hydratorMock = $this->getMockBuilder(HydratorInterface::class)