Issue #1862202 by plach, Berdir, katbailey, ParisLiakos, alexpott, chx, sun, larowlan, Gábor Hojtsy, cosmicdreams, vijaycs85, YesCT, penyaskito, andypost, Albert Volkman, joelpitett: Objectify the language system.
Revert "Issue #2171071 by jessebeach, Wim Leers: Remove drupal_add_library()."

Somehow this broke PHPUnit, so rolling back for now.

This reverts commit 91b465bdc5ad880b8a864e3e711d61c0cc0e5247.
Merge branch '8.x' of http://git.drupal.org/project/drupal into 8.x
Issue #2073535 by Wim Leers: Node history markers require an HTTP request .
Issue #2175447 by tim.plunkett, dawehner: Update composer to latest version.
Merge branch '8.x' of http://git.drupal.org/project/drupal into 8.x