In b90593333c3b0c4221619e2c00edc514af9a22eb the class \MediaWiki\Message\Message was moved from includes/Message/Message.php to includes/language/Message/Message.php as part of T364652.
The new location does not allow the class to autoload via PSR-4 (T353458: Make core classes follow PSR-4)
The class needs to be re-namespaced (T409441: Remove hierarchy, or re-namespace various classes) to \MediaWiki\Language\Message\Message.
Also remove the skip part for this namespace from AutoLoaderTest::testPsr4Mapping (ref 8d01c6d959)
This also affects the following classes in the same folder structure:
- MessageFormatterFactory.php
- TextFormatter.php
- Hook/MessagePostProcessHtmlHook.php
- Hook/MessagePostProcessTextHook.php