refactor: migrate index to ES2015 (#3006)

lib/index.js

@@ -1,20 +1,20 @@
-// index module
+'use strict'
 
-var constants = require('./constants')
-var Server = require('./server')
-var runner = require('./runner')
-var stopper = require('./stopper')
-var launcher = require('./launcher')
-var cfg = require('./config')
+const constants = require('./constants')
+const Server = require('./server')
+const runner = require('./runner')
+const stopper = require('./stopper')
+const launcher = require('./launcher')
+const cfg = require('./config')
 
 // TODO: remove in 1.0
-var oldServer = {
+const oldServer = {
   start: function (cliOptions, done) {
     console.error('WARN `start` method is deprecated since 0.13. It will be removed in 0.14. Please use \n' +
       '  server = new Server(config, [done])\n' +
       '  server.start()\n' +
       'instead.')
-    var server = new Server(cliOptions, done)
+    const server = new Server(cliOptions, done)
     server.start()
   }
 }

test/unit/index.spec.js

@@ -1,7 +1,9 @@
-var cfg = require('../../lib/config')
+'use strict'
+
+const cfg = require('../../lib/config')
 
 describe('index', () => {
-  var index = require('../../lib/index')
+  const index = require('../../lib/index')
 
   it('should expose the `config` object', () => {
     expect(index.config.parseConfig).to.be.eq(cfg.parseConfig)