true down cast using Proxy API.js

// from https://gist.github.com/613924
Proxy._noopHandler = function noopHandler(obj) {
  return {
    getOwnPropertyDescriptor: function(name) {
      var desc = Object.getOwnPropertyDescriptor(obj, name);
      // a trapping proxy's properties must always be configurable
      desc.configurable = true;
      return desc;
    },
    getPropertyDescriptor:  function(name) {
      var desc = Object.getPropertyDescriptor(obj, name); // not in ES5
      // a trapping proxy's properties must always be configurable
      desc.configurable = true;
      return desc;
    },
    getOwnPropertyNames: function() {
      return Object.getOwnPropertyNames(obj);
    },
    getPropertyNames: function() {
      return Object.getPropertyNames(obj);                // not in ES5
    },
    defineProperty: function(name, desc) {
      Object.defineProperty(obj, name, desc);
    },
    delete:       function(name) { return delete obj[name]; },
    fix:          function() {
      if (Object.isFrozen(obj)) {
        return Object.getOwnPropertyNames(obj).map(function(name) {
          return Object.getOwnPropertyDescriptor(obj, name);
        });
      }
      // As long as obj is not frozen, the proxy won't allow itself to be fixed
      return undefined; // will cause a TypeError to be thrown
    },

    has:          function(name) { return name in obj; },
    hasOwn:       function(name) { return ({}).hasOwnProperty.call(obj, name); },
    get:          function(receiver, name) { return obj[name]; },
    // bad behavior when set fails in non-strict mode
    set:          function(receiver, name, val) { obj[name] = val; return true; },
    enumerate:    function() {
      var result = [];
      for (name in obj) { result.push(name); };
      return result;
    },
    keys: function() { return Object.keys(obj); }
  };  
};

var _cast = function(obj, Class)
{
  if(!(obj instanceof Class))
    return null;
  
  var proxy = Proxy._noopHandler(obj)
  
  proxy.get = function(r, name)
  {
    var v = Class.prototype[name];
    
    if(v instanceof Function)
      return v.bind(obj);
    else
      return obj[name];
  }
  
  return Proxy.create(proxy, Class.prototype);
}; 

var A = function(a)
{
  this.a = a;
};

A.prototype._fn = function()
{
  console.log(this.a, 'A');
};

var B = function(a)
{
  A.call(this, a);
};

B.prototype = Object.create(A.prototype);
B.prototype.constructor = B;

B.prototype._fn = function()
{
  console.log(this.a, 'B');
};

var b = new B('test');
var a = _cast(b, A);

b._fn(); // test B
a._fn(); // test A
console.log(a.a); // test
console.log(a instanceof A); // true
console.log(_cast(b, Function) + ''); // null