spec: add helper functions for async/await tests

spec/events-helpers.js

@@ -0,0 +1,29 @@
+/**
+ * @fileoverview A set of helper functions to make it easier to work
+ * with events in async/await manner.
+ */
+
+/**
+ * @param {!EventTarget} target
+ * @param {string} eventName
+ * @return {!Promise<!Event>}
+ */
+const waitForEvent = (target, eventName) => {
+  return new Promise(resolve => {
+    target.addEventListener(eventName, resolve, {once: true})
+  })
+}
+
+/**
+ * @param {!EventEmitter} emitter
+ * @param {string} eventName
+ * @return {!Promise<!Array>} With Event as the first item.
+ */
+const emittedOnce = (emitter, eventName) => {
+  return new Promise(resolve => {
+    emitter.once(eventName, (...args) => resolve(args))
+  })
+}
+
+exports.emittedOnce = emittedOnce
+exports.waitForEvent = waitForEvent