Issue #1959660 by larowlan, pfrenssen, xjm, clemens.tolboom, dawehner, RobLoach: Replace xpath() with WebTestBase::cssSelect() by leveraging Symfony CssSelector.

Issue #1851234 by alexpott: Slow yaml parsing slows down tests which load/save many config files (e.g. views).

This caused a conflict in the language objectification patch. Postponing.

This reverts commit 9e017ada.

Issue #2151829 by dawehner, chx: Doctrine annotation parsing takes an unacceptable amount of time/memory on install.

Issue #1839468 by ParisLiakos, damiankloip, dawehner, jibran: Replace aggregator rss parsing with Zend Feed.

Issue #1894002 by scor, msonnabaum, effulgentsia: Update vendor libraries and pin them to specific versions in composer.json.

Issue #1849564 by attiks, Rob Loach, fago: Add the symfony validator component to core despite Symfony potentially releasing BC-breaking updates after 2.3.

Caused a conflict with a higher-priority patch; temporary rollback for now.

This reverts commit ba6d3fb7.

Issue #1834594 by effulgentsia, Rob Loach: Update dependencies (Symfony and Twig) follow up fixes for Composer.

Issue #1447736 by rbayliss, mikeytown2, effulgentsia, kim.pepper, Fabianx, boombatower: Adopt Guzzle library to replace drupal_http_request().

Issue #1810472 by linclark, mradcliffe: add Symfony's Serializer component to core despite Symfony potentially releasing BC-breaking updates after 2.3.