Fetch polyfill not working with Safari Technology Preview

Using Safari Technology Preview Version 9.1.1 (11601.6.10, 11602.1.25), when I try using fetch:

fetch('/')

It returns a rejected promise:

Promise {status: "rejected", result: "Fetch is not yet implemented"}

[dgraham]

https://twitter.com/jonathandavis/status/715315412791988225

[mislav]

BTW you can force the polyfill taking place if you nulify window.fetch before loading the polyfill:

<script>
  if (detectSafariPreview()) window.fetch = undefined
</script>
<script src="./fetch.js"></script>