cannot set property on fetch response

in the browser this works

let o = await fetch('https://deno.com');
o.asdf = 2
console.log(o.asdf)//2

let o = await fetch('https://deno.com');
o.asdf = 2
console.log(o.asdf)//2

let o = await fetch('https://deno.com');
o.asdf = 2
console.log(o.asdf)//2

let o = await fetch('https://deno.com');
o.asdf = 2
console.log(o.asdf)//2

but not in deno js