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Composer is a dependency management tool for PHP which Drupal 8 has adopted it to handle its PHP dependencies, such as Symfony and Twig. You will find composer.json available at /composer.json, which follows a schema in order to define the version dependencies for each required package.
drush dl composer
Alternatively, you can install Composer using straight-up PHP:
curl -s https://getcomposer.org/installer | php
Once you have Composer installed, you can update /composer.json accordingly, and run:
cd core drush composer update
Composer will retrieve new package information and update all the dependencies.