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Twig 1.22.0 introduces the concept of cache classes so we don't have to override as much of \Twig_Environment, but it also breaks our overrides.
Upgrade Twig to 1.22.0 and introduce our own cache class in this issue. Twig 2.x compatibility will be handled in.
Review, address feedback, commit.
User interface changes
There will be some changes to \Drupal\Core\Template\TwigEnvironment which hopefully nobody is relying on but we've seen in contrib people extending that instead of the vendor \Twig_Environment for no good reason. In general those shouldn't break, people would have to be relying on methods we will be removing around caching, storage, etc.
Data model changes
PASSED: [[SimpleTest]]: [PHP 5.5 MySQL] 113,576 pass(es). View
PASSED: [[SimpleTest]]: [PHP 5.5 MySQL] 113,542 pass(es). View