title: babel & karma & webpack not working
body: <p>I'm using babel with webpack, and test with karma and jasmine. 
  In order to use <code>Set()</code> (requires a polyfill), I've added :</p>
  
  <p><code>loader: 'babel-loader?optional[]=runtime'</code> </p>
  
  <p>to the configuration.</p>
  
  <p>This work's perfectly for the application itself, but the tests are not working, throwing: </p>
  
  <p><code>TypeError: 'undefined' is not an object (evaluating '_core.Object')</code></p>