Make Drupal 8 compatible with PHP 7.4

Problem/Motivation

Drupal 8.8.x is not passing tests on PHP 7.4. PHP 7.4 released on November 28, 2019, a week before Drupal 8.8.0.

Proposed resolution

Dependencies to be updated upstream

• Get a new release of easyrdf including https://github.com/njh/easyrdf/pull/311, see comment #28. Alternatively, remove dependency from easyrdf #3090017: Isolate test dependency on easyrdf/easyrdf to a single trait.

Dependencies to be updated in Drupal

• None at the moment.

Spun-off issues

• #3090145: Ensure that mixing array and Attribute objects in theme rendering is managed properly

Done:

• #3104420: PHP 7.4 Deprecated curly brace syntax for accessing array elements
• #3104421: PHP 7.4 deprecated reverse order of glue and pieces in implode
• #3095895: FieldTypePluginManager::getPluginClass() should throw an exception when called for a field type that doesn't exist
• #2830631: Remove code that tries to use _raw_variables for route argument resolution as it does not work
• #3089764: Updating translations does not clear the JS asset cache so new translations are not loaded
• Get a new release of composer/composer including https://github.com/composer/composer/pull/8296, see comment #33. #3075785: Update composer/composer to ^1.9.1
• Update to release 1.6.8 of mikey179/vfsstream, see comment #31. mikey179/vfsstream 2.0.x-dev requires PHP 7.2, so D8.8. cannot upgrade to it. #3091225: Require-dev mikey179/vfsstream ^1.6.8 in order to support PHP 7.4
• Get a new release of symfony/validator, including https://github.com/symfony/symfony/pull/34097:#3092112: Update symfony packages to 3.4.33 before 8.8.x beta
• #3089530: Tests of copied Doctrine code are not testing what we want them to test
• New release of Typo3's phar stream wrapper and of PEAR's Archive Tar #3088853: Require typo3/phar-stream-wrapper ^3.1.3 and pear/archive_tar ^1.4.8 in order to support PHP 7.4
• PHPunit 6 does not run on PHP 7.4 so we need to upgrade it - #2950132: Support PHPUnit 7 optionally in Drupal 8, while keeping support for ^6.5 - unfortunately PHP 7.0 (still supported in 8.8.0) does not run PHPUnit 7.
• egulias/email-validator - access array on null. Solvable updating to latest version. #3039611: Update core PHP dependencies for 8.8.x
• mikey179/vfsstream - use of deprecated curly brackets for array access. Solvable updating to latest version.
  #3039611: Update core PHP dependencies for 8.8.x