Issue #1393392 by c960657, fubhy: Convert prefix cache clears to cache tags, then remove support for them.

Issue #1669902 by alexpott, aspilicious, sun: Adjust system.performance configuration for new guidelines.

Issue #1711070 by kid_icarus, tim.plunkett, xjm, droplet, zendoodles and cam8001: Convert tests to use ::$modules property instead of parent::setUp($modules).

Issue #1706878 by tim.plunkett: Add WebTestBase::assertThemeOutput() to allow modules to test theme function output.

Issue #1496542 by cosmicdreams, sun, chx, vasi1186, aspilicious, alexpott, nadavoid, Pedro Lozano, disasm, andypost | heyrocker: Convert site information to config system.

Issue #1538478 by Niklas Fiekas: Register lib/ directories of themes as PSR-0 roots for Drupal$theme namespace.