fix: Mark `prototype` property of constructors as `readonly`

[ExE-Boss]

This more closely matches runtime behaviour.

---

It also causes code that would try to re‑assign the prototype property of WebIDL constructors to have the error caught at compile time, rather than at run time.

[orta]

We think this is probably a bit too big of a change for its value, I agree that it better reflects reality (example below) but it's hard to gauge a potential breaks from something like this.

AbortController.prototype = () => {}
> () => {}

new AbortController()
> AbortController {signal: AbortSignal}

const a = new AbortController()
undefined

a
>  AbortController { 
  signal: AbortSignal {aborted: false, onabort: null} 
  __proto__: AbortController
}

[saschanaz]

We think this is probably a bit too big of a change for its value, I agree that it better reflects reality (example below) but it's hard to gauge a potential breaks from something like this.

I'm not sure I understand the reasoning here, your example looks like it will benefit from readonly? The reason it's silently ignoring reassignation of .prototype is because it's not in strict mode.

"use strict"; AbortController.prototype = () => {}
> Uncaught TypeError: "prototype" is read-only

[saschanaz]

(But I guess some polyfill can legally assign things to .prototype)