get handler with Symbol

I fail to use the Proxy get handler with a ES6 Symbol as property name:

```
var Reflect = require('harmony-reflect');

var p = new Proxy(Object.create(null), {

    get: function(target, property) {

        return 'bar';
    }
});


console.log( p['foo'] );
console.log( p[Symbol('fooSym')] );
```
