In an article on yuiblog Douglas Crockford says that the for in statement will iterate over the methods of an object. Why does the following code not produce ["a", "b", "c", "d", "toString"]? Aren't .toString() and other methods members of my_obj?

Object.prototype.toString = function(){return 'abc'}
Object.prototype.d = 4;

my_obj = {
    'a':1,
    'b':2,
    'c':3
}

a = []
for (var key in my_obj) {
    a.push(key)
}

console.log(a) // prints ["a", "b", "c", "d"]