Make reactive-extra works on the phantomjs

Hi,

I had some issues when the meteor app loads inside the `phantomjs`
Here is the error
~~~
TypeError: 'undefined' is not a function (evaluating 'this._propertyGet.bind(this, name)')

  http://localhost:3000/packages/reactive-extra/lib/reactive-object.js?d652d3ba445563702ac2c35d4a4ddb583a45c08a:26
~~~

With this fix it works with PhantomJS.
I did this fix in JS. But I don't know how to write coffee :) 

Consider replicating this fix into coffee.