Use widget for comment subject field

Looks ok to me, lets just add the new migration to MigrateDrupal6Test.

Also a few nitpicks I noticed on the way through.

1. +++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/d6/CommentVariable.php
   @@ -59,6 +59,8 @@ protected function getCommentVariables() {
   +      $return[$node_type]['comment_type'] = empty($data['comment_subject_field']) ?
   +        'comment_no_subject' : 'comment';
   

   this could be on one line.

2. +++ b/core/modules/migrate_drupal/src/Tests/d6/MigrateCommentVariableField.php
   @@ -36,6 +36,20 @@ public function setUp() {
   +        ->save();
   

   bad indent.

Add D8MI topic tag.

Unable to speak about the migrate part, which is above my head atm, but the rest looks fine :-)

Only nitpick :

+++ b/core/modules/comment/src/Tests/CommentNonNodeTest.php
@@ -102,18 +102,20 @@ function postComment(EntityInterface $entity, $comment, $subject = '', $contact
+    if (entity_get_form_display('comment', 'comment', 'default')
+      ->getComponent('subject')) {

+++ b/core/modules/comment/src/Tests/CommentTestBase.php
@@ -113,18 +113,20 @@ public function postComment($entity, $comment, $subject = '', $contact = NULL, $
+    if (entity_get_form_display('comment', 'comment', 'default')
+      ->getComponent('subject')) {

Shoould be one line ?

Wow, so little code to make the change, so much test and migrate code.
Great work.

Re-rolled for #2293773: Field allowed values use dots in key names - not allowed in config

RTBC if re-roll comes back green.

Screenshots from manual testing

+1 rtbc

git ac https://www.drupal.org/files/issues/2292821-comment-subj-widget-29.patch
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 57857  100 57857    0     0   349k      0 --:--:-- --:--:-- --:--:--  376k
error: patch failed: core/modules/comment/src/Entity/Comment.php:224
error: core/modules/comment/src/Entity/Comment.php: patch does not apply

We're changing the UI and the API in this patch :)

Committed b7959a1 and pushed to 8.x. Thanks!