function setup(method)
{
  window[method] = fake;
};

var fake = function()
{
  return "faked";
};

var real = function()
{
  return "real";
};

setup("real");
console.log(real.toString());
console.log(real() == "faked");

/* this works but isn't as pretty */
function setup(method)
{
  return fake;
};
...
real = setup(real);
console.log(real() == "faked");