diff -u b/core/misc/message.es6.js b/core/misc/message.es6.js
--- b/core/misc/message.es6.js
+++ b/core/misc/message.es6.js
@@ -38,7 +38,7 @@
       if (!wrapper) {
         throw new Error(Drupal.t('There is no @element on the page.', { '@element': '[data-drupal-messages]' }));
       }
-      return wrapper.innerHTML === '' ? Drupal.theme('messageInternalWrapper', wrapper) : wrapper.firstElementChild;
+      return wrapper.innerHTML === '' ? Drupal.message.messageInternalWrapper(wrapper) : wrapper.firstElementChild;
     }
 
     /**
@@ -205,6 +205,22 @@
         Drupal.announce(options.announce || message, options.priority);
       }
     }
+
+    /**
+     * Function for creating the internal message wrapper element.
+     *
+     * @param {HTMLElement} messageWrapper
+     *   The message wrapper.
+     *
+     * @return {HTMLElement}
+     *   The internal wrapper DOM element.
+     */
+    static messageInternalWrapper(messageWrapper) {
+      const innerWrapper = document.createElement('div');
+      innerWrapper.setAttribute('class', 'messages__wrapper');
+      messageWrapper.insertAdjacentElement('afterbegin', innerWrapper);
+      return innerWrapper;
+    }
   };
 
   /**
@@ -250,18 +266,2 @@
   };
-
-  /**
-   * Theme function for internal message wrapper element.
-   *
-   * @param {HTMLElement} messageWrapper
-   *   The message wrapper.
-   *
-   * @return {HTMLElement}
-   *   The internal wrapper DOM element.
-   */
-  Drupal.theme.messageInternalWrapper = (messageWrapper) => {
-    const innerWrapper = document.createElement('div');
-    innerWrapper.setAttribute('class', 'messages__wrapper');
-    messageWrapper.insertAdjacentElement('afterbegin', innerWrapper);
-    return innerWrapper;
-  };
 })(Drupal);
diff -u b/core/misc/message.js b/core/misc/message.js
--- b/core/misc/message.js
+++ b/core/misc/message.js
@@ -84,7 +84,7 @@
         if (!wrapper) {
           throw new Error(Drupal.t('There is no @element on the page.', { '@element': '[data-drupal-messages]' }));
         }
-        return wrapper.innerHTML === '' ? Drupal.theme('messageInternalWrapper', wrapper) : wrapper.firstElementChild;
+        return wrapper.innerHTML === '' ? Drupal.message.messageInternalWrapper(wrapper) : wrapper.firstElementChild;
       }
     }, {
       key: 'getMessageTypes',
@@ -106,6 +106,14 @@
           Drupal.announce(options.announce || message, options.priority);
         }
       }
+    }, {
+      key: 'messageInternalWrapper',
+      value: function messageInternalWrapper(messageWrapper) {
+        var innerWrapper = document.createElement('div');
+        innerWrapper.setAttribute('class', 'messages__wrapper');
+        messageWrapper.insertAdjacentElement('afterbegin', innerWrapper);
+        return innerWrapper;
+      }
     }]);
 
     return _class;
@@ -138,9 +146,2 @@
   };
-
-  Drupal.theme.messageInternalWrapper = function (messageWrapper) {
-    var innerWrapper = document.createElement('div');
-    innerWrapper.setAttribute('class', 'messages__wrapper');
-    messageWrapper.insertAdjacentElement('afterbegin', innerWrapper);
-    return innerWrapper;
-  };
 })(Drupal);
\ No newline at end of file