ContextFactory::getInstance

  /**
   * Implements FactoryInterface::getInstance().
   */
  public function getInstance($options) {
    $config = $this->getConfiguration($options);
    $plugin_class = $this->getPluginClass($config);

    // Lets figure out of there's a constructor for this class and pull
    // arguments from the $options array if so to populate it.
    $reflector = new ReflectionClass($plugin_class);
    if ($reflector->hasMethod('__construct')) {
      $params = $reflector->getMethod('__construct')->getParameters();
      $args = array();
      foreach ($params as $param) {
        $args[$param->name] = $options[$param->name];
      }
      $instance = $reflector->newInstanceArgs($args);
    }
    else {
      $instance = new $plugin_class();
    }

    if ($instance instanceof PluginDerivativeInterface && isset($options['derivative'])) {
      $instance->getDerivative($options['derivative']);
    }
    if ($instance instanceof PluginContextInterface) {
      // @TODO: Once the Symfony Request component is in, we can revisit this.
      /*$container = drupal_container();
      $instance->setContext($container['context']);*/
    }
    return $instance;
  }