close() {
		            if (!IsWritableStream(this)) {
		                return promiseRejectedWith(streamBrandCheckException$2('close'));
		            }
		            if (IsWritableStreamLocked(this)) {
		                return promiseRejectedWith(new TypeError('Cannot close a stream that already has a writer'));
		            }
		            if (WritableStreamCloseQueuedOrInFlight(this)) {
		                return promiseRejectedWith(new TypeError('Cannot close an already-closing stream'));
		            }
		            return WritableStreamClose(this);
		        }
Consume and convert an entire Body to a Buffer.

Ref: https://fetch.spec.whatwg.org/#concept-body-consume-body

@return Promise