Object.freezing proxies should freeze or throw?

[Raul-Sebastian Mihăilă]

Initially posted on github (https://github.com/tc39/ecma262/issues/652),
but I think the mailing list is more appropriate.

Usually methods on Object that change some internal state of the
objects/properties of the objects either succeed or throw. However,
Object.freeze can fail to freeze proxies without throwing:

```js
      const target = Object.seal({x: 2});
      const proxy = new Proxy(target, {
        defineProperty() {
          return true;
        },

        set(target, prop, value) {
          target[prop] = value;
        }
      });

      Object.freeze(proxy);
      console.log(proxy.x); // 2
      proxy.x = 100;
      console.log(proxy.x); // 100
```

Should this be allowed?
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