Issue #2448843 by dawehner: [regression] Themes unable to implement hook_element_info_alter()

core/core.services.yml

core/lib/Drupal/Core/Render/ElementInfoManager.php

core/lib/Drupal/Core/Theme/ActiveTheme.php

...es/system/src/Tests/Render/ElementInfoIntegrationTest.php

core/modules/system/tests/themes/test_theme/test_theme.theme

...tests/Drupal/Tests/Core/Render/ElementInfoManagerTest.php