Issue #2053489 followup by tim.plunkett, klausi: Standardize on \Drupal throughout core.

core/includes/install.core.inc

core/includes/menu.inc

core/includes/theme.inc

core/includes/update.inc

core/modules/block/block.install

core/modules/entity/entity.module

core/modules/forum/forum.module

core/modules/menu/menu.install

core/modules/menu/menu.module

core/modules/node/node.install

core/modules/statistics/statistics.module

core/modules/system/system.install

...dules/system/tests/modules/system_test/system_test.module

core/modules/toolbar/toolbar.install

core/modules/user/user.install

core/scripts/dump-database-d6.sh

core/scripts/dump-database-d7.sh

core/themes/bartik/bartik.theme

core/themes/seven/seven.theme