Unit Testing Javascript - Object is not a constructor

I'm trying to learn to test some javascript files. Using mocha and chai with webpack.

This is my test/karma.conf.js

var webpack = require('webpack');

module.exports = function (config) {
  config.set({
    browsers: [ 'PhantomJS' ],
    singleRun: true,
    frameworks: [ 'mocha' ],
    files: [
      'tests.webpack.js'
    ],
    preprocessors: {
      'tests.webpack.js': [ 'webpack', 'sourcemap' ]
    },
    reporters: [ 'spec' ],
    webpack: {
      devtool: 'inline-source-map',
      module: {
        loaders: [
          { test: /\.js$/, loader: 'babel-loader' }
        ]
      }
    },
    webpackServer: {
      noInfo: true
    }
  });
};

And this the test/test.webpack.js which I use on the karma config:

import chai from 'chai'

global.chai = chai
global.expect = chai.expect
global.should = chai.should()

var context = require.context('../src', true, /spec\.js$/);
context.keys().forEach(context);

my test is very basic, just in order to test if it works src/index.spec.js:

describe("A test suite", function() {
   beforeEach(function() { });
   afterEach(function() { });
   it('should fail', function() { expect(true).to.be.true; });
});

When I try to run the tests, I got this error

./node_modules/karma/bin/karma start test/karma.conf.js

01 04 2016 14:15:08.191:INFO [karma]: Karma v0.13.22 server started at http://localhost:9876/
01 04 2016 14:15:08.255:INFO [launcher]: Starting browser PhantomJS
01 04 2016 14:15:12.443:INFO [PhantomJS 2.1.1 (Windows 8 0.0.0)]: Connected on socket /#GivXNjnm0g5H9DBXAAAA with id 29314805

PhantomJS 2.1.1 (Windows 8 0.0.0) ERROR
  TypeError: Object is not a constructor (evaluating '$export($export.S + $export.F * !__webpack_require__(17), 'Object', { defineProperty: __webpack_require__(13).f })')
  at C:/Users/pablo.feldman/Documents/Projects/jpmc-components/test/tests.webpack.js:535 <- webpack:///~/babel-runtime/~/core-js/library/modules/es6.object.define-property.js:3:0

PhantomJS 2.1.1 (Windows 8 0.0.0): Executed 0 of 0 ERROR (0.101 secs / 0 secs)