https://medium.com/@priyeshp18/push-notifications-between-ionic-2-3-apps-using-fcm-d3971b0120ce
Fix for error "The "chunk" argument must be one of type string or Buffer" during build:
https://github.com/fechanique/cordova-plugin-fcm/issues/481#issuecomment-376008472
Needs cordova-plugin-android-permissions:
$ionic cordova plugin add cordova-plugin-android-permissions