问如何用web组件测试仪测试用ES6/ES6 2015编写的聚合物组件？

EN

chrome 48                ✖ Test Suite Initialization

  Block-scoped declarations (let, const, function, class) not yet supported outside strict mode

chrome 48                Tests failed: 2 failed tests

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<link rel="import" href="../../bower_components/polymer/polymer.html">

<dom-module id="my-list">
  <template>
    <style>
      :host {
        display: block;
      }
    </style>
    <ul>
      <template is="dom-repeat" items="{{items}}">
        <li><span class="paper-font-body1">{{item}}</span></li>
      </template>
    </ul>
  </template>
  <script>
    (() => {
      'use strict';

      class MyList {

        beforeRegister() {
          let is = this.constructor.name
            .replace(/\W+/g, '-')
            .replace(/([a-z\d])([A-Z])/g, '$1-$2')
            .toLowerCase();

          this.is = is;

          this.properties = {
            items : {
              type   : Array,
              notify : true,
            }
          };
        }

        ready() {
          this.items = [
            'Responsive Web App boilerplate',
            'Iron Elements and Paper Elements',
            'End-to-end Build Tooling (including Vulcanize)',
            'Unit testing with Web Component Tester',
            'Routing with Page.js',
            'Offline support with the Platinum Service Worker Elements'
          ];
        }
      }

      Polymer(MyList);
    })();
  </script>
</dom-module>