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In fetch step 9: "If request's synchronous flag is set, wait for either end-of-file to have been pushed to response's *body* or for response to have a termination reason, and then return response." response's body is pointing to #concept-request-body.
https://github.com/whatwg/fetch/commit/38f10d5db50833b4953d1855f92807cb8a5867ae
O, catch this earlier so now is correct: https://www.w3.org/Bugs/Public/show_bug.cgi?id=27507#c1