This is a sample application coded following this excellent tutorial: Push Notifications codelab.
This app has to do with Service Workers, Google Cloud Messaging (GCM) and push notifications
Just type this command on your terminal window:
curl --header "Authorization: key=<YOUR_GOOGLE_API_KEY>" --header "Content-Type: application/json" https://android.googleapis.com/gcm/send -d "{\"registration_ids\":[\"<YOUR_REGISTRATION_ID>\"]}"
You can find your google API key inside Credentials section in your Google Developers Console.
On the other hand, you can find YOUR_REGISTRATION_ID once your application performed a successfully subscription to Google Cloud Messaging.