Merge branch '8.x' of git.drupal.org:project/drupal into 8.x

core/includes/common.inc

core/includes/file.inc

core/includes/form.inc

core/lib/Drupal/Core/Cache/CacheBackendInterface.php

core/modules/field/field.module

.../field/modules/field_sql_storage/field_sql_storage.module

core/modules/help/help.module

core/modules/node/node.test

core/modules/poll/poll-results.tpl.php

core/modules/poll/poll-vote.tpl.php

core/modules/poll/poll.test

core/modules/simpletest/drupal_web_test_case.php

core/modules/system/system.module

core/modules/system/tests/form.test

core/modules/system/tests/modules/form_test/form_test.module