gistfile1.js

var function_sandbox = (function(global) {
  function SubFunc(func) {
    func.__proto__ = funcPlugin;
    return func;
  }

  var funcPlugin, __Function = global.Function;

  // make prototype values conform to ECMA spec and inherit from regular natives
  (SubFunc.prototype = new __Function('')).__proto__ = __Function.prototype;

  // assign constructor property
  (funcPlugin = SubFunc.prototype).constructor = SubFunc;

  return SubFunc;
})(this);

Usage:

function blah() { /*...*/ }
var foo = function_sandbox(blah);
foo(/*...*/); // works, and it's checkable if `foo` is overriden to not actually be `blah`