importScripts('https://www.gstatic.com/firebasejs/3.6.6/firebase-app.js');
importScripts('https://www.gstatic.com/firebasejs/3.6.6/firebase-messaging.js');

// Initialize the Firebase app in the service worker by passing in the
// messagingSenderId.
firebase.initializeApp({
  // TODO add your messagingSenderId
  messagingSenderId: '662518903527'
});
var messaging = firebase.messaging();

importScripts('https://www.gstatic.com/firebasejs/3.6.6/firebase-app.js');
importScripts('https://www.gstatic.com/firebasejs/3.6.6/firebase-messaging.js');

// Initialize the Firebase app in the service worker by passing in the
// messagingSenderId.
firebase.initializeApp({
  // TODO add your messagingSenderId
  messagingSenderId: ''
});
var messaging = firebase.messaging();

Remove hard-coded message sender id.