Don't add 'constructor' as a dependent key

I'm not sure if this is the proper fix for this issue, but we were running into a problem with a function like so:

```
className: ~> @object.constructor.toString()
```

This was generating a dependency on the property `object.constructor` which is not an Ember property. Instead, it seems like the dependency should be on the `object` itself in this case. Let me know if there's a better place to check for this at compile-time, or if you think this isn't actually an issue.