Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/comment/js/comment-by-viewer.js

@@ -0,0 +1,26 @@
+/**
+ * @file
+ * Attaches behaviors for the Comment module's "by-viewer" class.
+ */
+
+(function ($, Drupal, drupalSettings) {
+
+  "use strict";
+
+  /**
+   * Add 'by-viewer' class to comments written by the current user.
+   *
+   * @type {Drupal~behavior}
+   */
+  Drupal.behaviors.commentByViewer = {
+    attach: function (context) {
+      var currentUserID = parseInt(drupalSettings.user.uid, 10);
+      $('[data-comment-user-id]')
+        .filter(function () {
+          return parseInt(this.getAttribute('data-comment-user-id'), 10) === currentUserID;
+        })
+        .addClass('by-viewer');
+    }
+  };
+
+})(jQuery, Drupal, drupalSettings);