title: Use of reserved word 'let' when running tests
body: <p>Started getting this error I just can't seem to fix, when running my tests.</p>
  
  <pre><code>09 04 2017 22:08:20.577:INFO [karma]: Karma v1.6.0 server started at http://0.0.0.0:9876/
  09 04 2017 22:08:20.580:INFO [launcher]: Launching browser PhantomJS with unlimited concurrency
  09 04 2017 22:08:20.602:INFO [launcher]: Starting browser PhantomJS
  09 04 2017 22:08:23.661:INFO [PhantomJS 2.1.1 (Windows 8 0.0.0)]: Connected on socket SPM1D8Mj1w6ABbGuAAAA with id 7910462
  PhantomJS 2.1.1 (Windows 8 0.0.0) ERROR
    SyntaxError: Use of reserved word 'let' in strict mode
    at test/browser/index.js:886
  </code></pre>
  
  <p>I believe this started happening during certain packages updating.</p>
  
  <p>My Karma config looks like this :</p>
  
  <pre><code>require('babel-register');
  
  const webpack = require('../webpack.config.babel.js');
  
  module.exports = function(config) {
    config.set({
      basePath: '../',
      frameworks: ['mocha', 'chai-sinon'],
      browsers: ['PhantomJS'],
      files: ['test/browser/**/*.js'],
      preprocessors: {
        'test/**/*.js': ['webpack'],
        'src/**/*.js': ['webpack']
      },
      webpack,
      webpackMiddleware: { noInfo: true }
    });
  };
  </code></pre>
  
  <p>Tests are pretty simple </p>
  
  <pre><code>import { h, render, rerender } from 'preact';
  import { route } from 'preact-router';
  import App from '../../src/components/app';
  
  /*global sinon,expect*/
  describe('App', () =&gt; {
    var scratch;
  
    before(() =&gt; {
      scratch = document.createElement('div');
  
      (document.body || document.documentElement).appendChild(scratch);
    });
  
    beforeEach(() =&gt; {
      scratch.innerHTML = '';
    });
  
    after(() =&gt; {
      scratch.parentNode.removeChild(scratch);
    });
  
    describe('routing', () =&gt; {
      it('should render the homepage', () =&gt; {
        render(&lt;App /&gt;, scratch);
        expect(scratch.innerHTML).not.contain('Home');
      });
  
      it('should render the header', () =&gt; {
        render(&lt;App /&gt;, scratch);
        expect(scratch.innerHTML).to.contain('header');
      });
  
      it('should render the footer', () =&gt; {
        render(&lt;App /&gt;, scratch);
  
        expect(scratch.innerHTML).to.contain('footer');
      });
  
      it('should render the social media links', () =&gt; {
        render(&lt;App /&gt;, scratch);
  
        expect(scratch.innerHTML).to.contain('a');
      });
    });
  });
  </code></pre>
  
  <p>Any idea where I could even find out which "let" it is talking about ?</p>
  
  <p>I am running this on Node v7.8.0</p>