Adds a check to the registry, refresh the trigger to check map.

def RegisterCheck(cls, check, source="unknown", overwrite_if_exists=False):
    """Adds a check to the registry, refresh the trigger to check map."""
    if not overwrite_if_exists and check.check_id in cls.checks:
      raise DefinitionError(
          "Check named %s already exists and "
          "overwrite_if_exists is set to False." % check.check_id)
    check.loaded_from = source
    cls.checks[check.check_id] = check
    cls.triggers.Update(check.triggers, check)