Fix for `no-proto` and `no-iterator` false positive

This fixes an issue where both `no-proto` and `no-iterator` would incorrectly warn on the following code:

```js
var foo = bar[__iterator__];
```

Because `__iterator__` is a variable and not a property on `bar`, there should be no warning.

This issue was raised with this comment: https://github.com/nzakas/eslint/pull/275#issuecomment-23296294