External dependencies removed from the core repo and managed by Composer instead

The vendor directory of external dependencies has been removed from the Drupal core git repository for the 8.1.x and 8.2.x branches. This change improves Drupal 8's compatibility with a Composer-based workflow for sites.

There will not be any changes for people downloading Drupal 8 from Drupal.org. The Drupal.org packager will add dependencies to zip and tar packages.

If you're not using zip / tar files, e.g. when using a git clone, you must run composer install to get dependencies. This command can be executed on a new or existing site. See https://www.drupal.org/documentation/install/download#git for instructions.