Vue Unit Test Error: vuex requires a Promise polyfill in this browser

I created a project using vue-cli and added vuex and vue-router in it. I am trying to setup a unit test for it, but I am getting following error. Without Vuex, it used to work.

PhantomJS 2.1.1 (Mac OS X 0.0.0) ERROR

Error: [vuex] vuex requires a Promise polyfill in this browser.

at webpack:///~/vuex/dist/vuex.js:145:0 <- index.js:9871

The following are the relevant package versions:

"babel-core": "^6.0.0",
"babel-eslint": "^7.0.0",
"babel-loader": "^6.0.0",
"vue": "^2.1.0",
"vue-router": "^2.0.3",
"vuex": "^2.0.0",
"vuex-router-sync": "^3.0.0"
"karma": "^1.3.0",
"karma-coverage": "^1.1.1",
"karma-mocha": "^1.2.0",
"karma-phantomjs-launcher": "^1.0.0",
"karma-sinon-chai": "^1.2.0",
"karma-sourcemap-loader": "^0.3.7",
"karma-spec-reporter": "0.0.26",
"karma-webpack": "^1.7.0",
"webpack": "^1.13.2",
"webpack-dev-middleware": "^1.8.3",
"webpack-hot-middleware": "^2.12.2",
"webpack-merge": "^0.14.1"

The following is karma.conf.js:

// This is a karma config file. For more details see
//   http://karma-runner.github.io/0.13/config/configuration-file.html
// we are also using it with karma-webpack
//   https://github.com/webpack/karma-webpack

var path = require('path')
var merge = require('webpack-merge')
var baseConfig = require('../../build/webpack.base.conf')
var utils = require('../../build/utils')
var webpack = require('webpack')
var projectRoot = path.resolve(__dirname, '../../')

var webpackConfig = merge(baseConfig, {
  // use inline sourcemap for karma-sourcemap-loader
  module: {
    loaders: utils.styleLoaders()
  },
  devtool: '#inline-source-map',
  vue: {
    loaders: {
      js: 'isparta'
    }
  },
  plugins: [
    new webpack.DefinePlugin({
      'process.env': require('../../config/test.env')
    })
  ]
})

// no need for app entry during tests
delete webpackConfig.entry

// make sure isparta loader is applied before eslint
webpackConfig.module.preLoaders = webpackConfig.module.preLoaders || []
webpackConfig.module.preLoaders.unshift({
  test: /\.js$/,
  loader: 'isparta',
  include: path.resolve(projectRoot, 'src'),
  exclude: /test\/unit|node_modules/
})

// only apply babel for test files when using isparta
webpackConfig.module.loaders.some(function (loader, i) {
  if (loader.loader === 'babel') {
    loader.include = path.resolve(projectRoot, 'test/unit')
    return true
  }
})

module.exports = function (config) {
  config.set({
    // to run in additional browsers:
    // 1. install corresponding karma launcher
    //    http://karma-runner.github.io/0.13/config/browsers.html
    // 2. add it to the `browsers` array below.
    browsers: ['Chrome'],
    frameworks: ['mocha', 'sinon-chai'],
    reporters: ['spec', 'coverage'],
    files: ['./index.js'],
    preprocessors: {
      './index.js': ['webpack', 'sourcemap']
    },
    webpack: webpackConfig,
    webpackMiddleware: {
      noInfo: true
    },
    coverageReporter: {
      dir: './coverage',
      reporters: [
        { type: 'lcov', subdir: '.' },
        { type: 'text-summary' }
      ]
    }
  })
}

Using Babel polyfill solved the problem. Here are the steps what I did:

Installing Babel Polyfill:

npm install --save-dev babel-polyfill

then include the polyfill file before your source and test files within the files section of your karma.conf.js:

files: [
  '../node_modules/babel-polyfill/dist/polyfill.js',
  'index.js'
],