ECMAScript 2015 (ES6) is supported in Node.js v8.9.4 and the later versions, so you may apply it in Cloud Functions. An overview over supported ECMAScript features in various versions of Node.js is provided by node.green, which shows all shipping features (The ones which are turned on by default on Node.js and do NOT require any kind of runtime flag).
Here you could find a sample on how to publish messages to a Cloud Pub/Sub topic from a Cloud Function using ES6.