Issue #1982984 by larowlan: Create Drupal::entityManager for improved DX.

core/includes/common.inc

core/includes/config.inc

core/includes/entity.inc

core/includes/menu.inc

core/lib/Drupal.php

core/lib/Drupal/Core/Entity/Entity.php

core/lib/Drupal/Core/Entity/EntityNG.php

core/lib/Drupal/Core/Entity/Field/Type/EntityTranslation.php

core/lib/Drupal/Core/Entity/Field/Type/EntityWrapper.php

core/modules/block/block.admin.inc

core/modules/block/custom_block/custom_block.admin.inc

core/modules/config/tests/config_test/config_test.module

core/modules/contact/contact.admin.inc

...dules/field/lib/Drupal/field/Plugin/Core/Entity/Field.php

...eld/lib/Drupal/field/Plugin/Core/Entity/FieldInstance.php

core/modules/file/file.module

.../modules/menu/lib/Drupal/menu/Form/MenuDeleteMenuForm.php

core/modules/menu/menu.admin.inc

...menu_link/lib/Drupal/menu_link/MenuLinkFormController.php

...link/lib/Drupal/menu_link/Plugin/Core/Entity/MenuLink.php