ServiceProvider class for mandatory core services.
This is where Drupal core registers all of its compiler passes. The service definitions themselves are in core/core.services.yml with a few, documented exceptions (typically, install requirements).
Modules wishing to register services to the container should use modulename.services.yml in their respective directories.
Hierarchy
class \Drupal\Core\CoreServiceProvider implements ServiceModifierInterface, ServiceProviderInterface
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