Constructs a BlockPluginBag object.
\Drupal\Component\Plugin\PluginManagerInterface $manager: The manager to be used for instantiating plugins.
array $instance_ids: The ids of the plugin instances with which we are dealing.
\Drupal\block\Plugin\Core\Entity\Block $entity: The Block entity that holds our configuration.
public function __construct(PluginManagerInterface $manager, array $instance_ids, Block $entity) {
$this->manager = $manager;
$this->entity = $entity;
$this->instanceIDs = drupal_map_assoc($instance_ids);
}