Ember 1.12 does not work on IE11

I upgraded my Ember 1.8 webapp to Ember 1.12. On 1.8 it worked on all browsers, on 1.12 it does not work anymore on IE11. I get SCRIPT1014 (invalid character) on:

 let CONTAINER = `__${new Date()}_container`;

And I get SCRIPT5009 (define is not defined) on:

 define('xxx/adapters/yyy', ['exports', 'xxx/adapters/base/zzz-adapter'], function (exports, zzz) {

Upgrading to Ember 1.13 is not an option on the short term. Anyone else having problems with Ember 1.12 and IE11?