The GitHub blob onebox does not highlight the `.js.es6` code

Feature
1

https://github.com/discourse/discourse/blob/v2.0.0.beta4/app/assets/javascripts/discourse/routes/application.js.es6#L56-L69

The same code pasted directly into the editor is highlighted:

composePrivateMessage(user, post) {

  const recipient = user ? user.get('username') : '',
      reply = post ? window.location.protocol + "//" + window.location.host + post.get("url") : null;

  // used only once, one less dependency
  return this.controllerFor('composer').open({
    action: Composer.PRIVATE_MESSAGE,
    usernames: recipient,
    archetypeId: 'private_message',
    draftKey: 'new_private_message',
    reply: reply
  });
},