Object.freezing proxies should freeze or throw?

[Claude Pache]

Given a Proxy that pretends to be in state A while its target is observably in state B, and assuming that the target satisfies the Invariants of the Essential Internal Methods [6.1.7.3], I claim that, in order to force the Proxy to satisfy those Invariants, it is necessary and sufficient to check that the two following conditions hold:

* it is legal for an object to pass from state A to state B; and,
* it is legal for an object to pass from state B to state A.

[6.1.7.3]: https://tc39.github.io/ecma262/#sec-invariants-of-the-essential-internal-methods


Because I am too lazy to write the proof just now, I cowardly leave it as an exercice to the reader. Meanwhile, that principle may be used to audit the robustness of the Proxy specification. I have found the following bug in Proxy.[[Delete]]() by applying the above principle to:

* state A: nonexistent property on a nonextensible object;
* state B: existent own property on a nonextensible object.

Resurrection of a successfully deleted property on a nonextensible object:

```js
var target = Object.preventExtensions({ x: 1 })
var proxy = new Proxy(target, { 
    deleteProperty() { return true } 
})

Object.isExtensible(proxy) // false
delete proxy.x // true
proxy.hasOwnProperty('x') // true
```

After a first scan, I haven't found other bugs in the essential methods of Proxy, than that one and the missing nonconfigurable-but-writable check in [[GetOwnPropertyDescriptor]] and [[DefineOwnProperty]] already mentioned in that thread.

I plan to propose a minimal patch (i.e., just adding the missing checks) in a few days.

—Claude