According the the Angular core Slack, there is an issue in some browsers when trying to call `Symbol.for`. Namely in Safari 8 and all versions of IE. I haven't reproduced this yet, but I can see/understand where the issue is. Such is the danger with DIY polyfills. You can see that [Symbol is supported](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol) in more browsers than [Symbol.for is](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/for).