lib.d.ts has a wrong definition of HeadersInit

TypeScript Version: 2.6.1 (and 2.7.0-dev.20171101)

Code

fetch('/some-url', { headers: { 'Some-Header-Name': 'Value' } });

Expected behavior:
No errors.

Actual behavior:
Error:

Object literal may only specify known properties, and ''Some-Header-Name'' does not exist in type 'Headers | string[][]'.

According the spec RequestInit may has headers prop as HeadersInit type which is

type HeadersInit = [string, string][] | { [key: string]: string };

But currently we have Headers instead of map object in RequestInit.headers (the code above works fine in all browsers (including Edge)).

Instead of:

fetch('/some-url', { headers: { 'Some-Header-Name': 'Value' } });

Try:

let headers: any = { headers: { 'Some-Header-Name': 'Value' } };

fetch('/some-url', headers);

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