The Object.defineProerty shim had a bogus check for browser implementation and this change fixes it.

This well be our first patch :) It's small but we'd like to get familiar with the process.

This bug is [well](http://code.google.com/p/box2dweb/issues/detail?id=41) [known](http://code.google.com/p/box2dweb/issues/detail?id=43) to be a bug in Box2D itself, but as our `cc.Class.extend` patch would require this, let's fix this first. I hope this is not adding maintenance burden on this project.