proposal: Object Members

[Darien Valentine]

> That, and that the existing builtins already impose those limitations -
and only class allows you to do those things, with them.

I’m surprised by that statement — it appeared to me that it currently
remains possible to create classes, including classes that extend
built-ins, with class syntax.

```
const test = value => {
  if (typeof value !== 'string') throw new TypeError('nope');
  return value;
};

function StringSet(init) {
  if (new.target === undefined)
    throw new TypeError('StringSet is not a constructor');
  if (init !== undefined)
    init = Array.from(init, test);

  return Reflect.construct(Set, [ init ], new.target);
}

Reflect.defineProperty(StringSet.prototype, 'add', {
  configurable: true,
  value: Object.setPrototypeOf({
    add(value) {
      return super.add(test(value));
    }
  }, Set.prototype).add
});

Reflect.defineProperty(StringSet.prototype, Symbol.toStringTag, {
  configurable: true,
  value: 'StringSet'
});

Reflect.setPrototypeOf(StringSet, Set);
Reflect.setPrototypeOf(StringSet.prototype, Set.prototype);

new StringSet('abc');
```

Not that this is something one is apt to want to do normally, though it
being possible does remain useful sometimes for meta/compositiony stuff. It
cannot be achieved in an <= ES5 environment, but it seems all the
reflection tools needed are present in environments that actually have
class syntax — even, despite the awkwardness necessitated by HomeObject
stuff, what is needed to employ super in methods.

I’m curious if, aside from the possibility of implementation-specific
things like type error messages being different, there there is anything
about the above class which ends up observably different from the ES-side
from one created with class syntax instead?
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