Hide ReactElement constructor

This prevents feature tests like:

```javascript
var ReactElement = React.createElement(...).constructor;

if (element.constructor === ReactElement) ...

// or

if (element.constructor !== Object) ...
```

This is intentional so that we have the option to make these plain objects. E.g. for direct inlining or replacing them with value types.

I suspect that people will be tempted to use this hack. E.g. to avoid passing null props. Therefore I'm adding this PR to 0.12 last minute.