Issue #1812866 by mkadin, effulgentsia: rebuild the server side AJAX API.

core/lib/Drupal/Core/Ajax/AddCssCommand.php

core/lib/Drupal/Core/Ajax/AfterCommand.php

core/lib/Drupal/Core/Ajax/AjaxResponse.php

core/lib/Drupal/Core/Ajax/AlertCommand.php

core/lib/Drupal/Core/Ajax/AppendCommand.php

core/lib/Drupal/Core/Ajax/BeforeCommand.php

core/lib/Drupal/Core/Ajax/ChangedCommand.php

core/lib/Drupal/Core/Ajax/CommandInterface.php

core/lib/Drupal/Core/Ajax/CssCommand.php

core/lib/Drupal/Core/Ajax/DataCommand.php

core/lib/Drupal/Core/Ajax/HtmlCommand.php

core/lib/Drupal/Core/Ajax/InsertCommand.php

core/lib/Drupal/Core/Ajax/InvokeCommand.php

core/lib/Drupal/Core/Ajax/PrependCommand.php

core/lib/Drupal/Core/Ajax/RemoveCommand.php

core/lib/Drupal/Core/Ajax/ReplaceCommand.php

core/lib/Drupal/Core/Ajax/RestripeCommand.php

core/lib/Drupal/Core/Ajax/SettingsCommand.php

core/modules/file/file.module

...tem/lib/Drupal/system/Tests/Ajax/AjaxCommandsUnitTest.php