Add "app.whenReady()" (#12652)

* Make "chai-as-promised" avaialble in tests

* Add "app.whenReady()"

Closes #9561.

spec/api-app-spec.js

@@ -1,4 +1,6 @@
 const assert = require('assert')
+const chai = require('chai')
+const chaiAsPromised = require('chai-as-promised')
 const ChildProcess = require('child_process')
 const https = require('https')
 const net = require('net')
@@ -7,10 +9,13 @@ const path = require('path')
 const {ipcRenderer, remote} = require('electron')
 const {closeWindow} = require('./window-helpers')
 
+const {expect} = chai
 const {app, BrowserWindow, Menu, ipcMain} = remote
 
 const isCI = remote.getGlobal('isCi')
 
+chai.use(chaiAsPromised)
+
 describe('electron module', () => {
   it('does not expose internal modules to require', () => {
     assert.throws(() => {
@@ -903,4 +908,15 @@ describe('app module', () => {
       v8Util.requestGarbageCollectionForTesting()
     })
   })
+
+  describe('whenReady', () => {
+    it('returns a Promise', () => {
+      expect(app.whenReady()).to.be.an.instanceOf(Promise)
+    })
+
+    it('becomes fulfilled if the app is already ready', () => {
+      assert(app.isReady())
+      return expect(app.whenReady()).to.be.eventually.fulfilled
+    })
+  })
 })