[BUGFIX beta] Watching non-configurable properties

If you try to re-define property that is non-configurable, the error
is going to be thrown "You can't re-define property X".

To illustrate the issue:

```javascript
var obj = {};

Object.defineProperty(obj, 'name', {});

// the following code throws exception
Object.defineProperty(obj, 'name', {
  configurable: true
});
```

According to
[MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty?redirectlocale=en-US&redirectslug=JavaScript%2FReference%2FGlobal_Objects%2FObject%2FdefineProperty)
when you define property without specifying the descriptor,
`configurable` defaults to `false`.

This PR adds an assertion if you're trying to re-define non-configurable
properties.