Fetch polyfill has different Response-object in internet explorer 11

I'm currently struggeling with Internet Explorer 11 and the Fetch API.

I have the following fetch-call unsing TypeScript:

await fetch(theForm.action, { method: theForm.method, body: formData, credentials: "include" })
    .then((response: Response) => {
        if (response.status === 500)
            throw Error(response.statusText);

        if (response.redirected && response.url)
            return window.location.href = response.url;

        return response.json();
    })
    .catch((error: Error) => {
        return this.setState({ isSubmitting: false, internalServerError: error });
    });

The code is ts-compiled to ES2015 and then down to ES5 using babel. I've also added babel-polyfill. The example code works as expected on other browsers than IE. As it turns out, the Response-object in IE is totally different and don't even has the property redirected and url is always empty.

Is there any polyfill I should use to make this work in IE? And, if not, what would be a good workaround?