Why is webpack outputting the const keyword instead of var?

Question

Webpack appears to be outputting the const keyword instead of the var keyword which is breaking PhantomJS (as it doesn't support ES6 syntax).

...    

/* 2 */
/***/ function(module, exports, __webpack_require__) {    
    "use strict";
    const testing_1 = __webpack_require__(3); // const keyword breaking PhantomJS
    testing_1.describe('App', () => {

So PhantomJS reports

PhantomJS 2.1.1 (Windows 8 0.0.0) ERROR
  SyntaxError: Unexpected token 'const'

The file in question (karma-shim.js) is being processed by the karma-webpack preprocessor

karma-shim.js

'use strict';

Error.stackTraceLimit = Infinity;
require('es6-shim');
require('angular2/bundles/angular2-polyfills.js');
require('angular2/testing');

var appContext = require.context('./src', true, /root\.spec\.ts/);

appContext.keys().forEach(appContext);

var testing = require('angular2/testing');
var browser = require('angular2/platform/testing/browser');

testing.setBaseTestProviders(browser.TEST_BROWSER_PLATFORM_PROVIDERS, browser.TEST_BROWSER_APPLICATION_PROVIDERS);

karma.conf.js

...
preprocessors: {
    './karma-shim.js': ['webpack', 'sourcemap']
},

tsconfig.json

{
    "compilerOptions": {
        "target": "ES5",
        "module": "commonjs",
        "emitDecoratorMetadata": true,
        "experimentalDecorators": true,
        "sourceMap": true,
        "moduleResolution": "node",
        "jsx": "react"
    },
    "exclude": [
        "node_modules"
    ],
    "compileOnSave": false,
    "buildOnSave": false
}

webpack.config.js Is part of generator-ng2-webpack (https://github.com/cmelion/generator-ng2-webpack) but doesn't appear to mention transpilation.

Why would webpack be outputting const instead of var and is there anything I can do about it to help PhantomJS?