Use own Map imlementation if size is getter

The problem is that in current spec, Map.prototype.size is a getter. So:
```javascript
var prototype = Set[prototype_],
        m = M[prototype_],
        msize = callbind(m.size),
        mforEach = callbind(m.forEach),
        mget = callbind(m.get),
        mset = callbind(m.set),
        mhas = callbind(m.has),
        mdelete = callbind(m['delete']);
```

Here you actually call the getter, so in ```traceur``` map implementations, it fails. Also, the same thing happens in Firefox. So, may be there is more elegant solution. But it is the fastest one. Generally, I think we can bind getter, the same way as simple function. But, I'm not sure it's the good way.

The initial problem occurred in one of Meteor.js plugins, after Meteor 1.0.2.1 release.

https://github.com/mquandalle/meteor-harmony/issues/37