function IsWritableStream(x) {
		        if (!typeIsObject(x)) {
		            return false;
		        }
		        if (!Object.prototype.hasOwnProperty.call(x, '_writableStreamController')) {
		            return false;
		        }
		        return x instanceof WritableStream;
		    }
The Fetch Standard treats this as if "total bytes" is a property on the body.
For us, we have to explicitly get it with a function.

ref: https://fetch.spec.whatwg.org/#concept-body-total-bytes

@param {any} obj.body Body object from the Body instance.
@returns {number | null}