Use of reserved word 'let' in strict mode

[Overdrivr]

If I import vue-octicon inside a component, I get the following error during testing (by running unit tests generated with vue-loader )

<script>
import Octicon from 'vue-octicon/components/Octicon'
import 'vue-octicon/icons'

export default {
  name: 'hello',
  components: {
     'octicon': Octicon
  },
  data () {
     return {}
  }
}

SyntaxError: Use of reserved word 'let' in strict mode
  at webpack:///node_modules/vue-octicon/util.js:3:0

Any idea on how to solve this ?