private function retrieveCertsFromLocation($url)
  {
    // If we're retrieving a local file, just grab it.
    if (0 !== strpos($url, 'http')) {
      if (!$file = file_get_contents($url)) {
        throw new Google_Exception(
            "Failed to retrieve verification certificates: '" .
            $url . "'."
        );
      }

      return json_decode($file, true);
    }

    $response = $this->http->get($url);

    if ($response->getStatusCode() == 200) {
      return json_decode((string) $response->getBody(), true);
    }
    throw new Google_Exception(
        sprintf(
            'Failed to retrieve verification certificates: "%s".',
            $response->getBody()->getContents()
        ),
        $response->getStatusCode()
    );
  }