About isNative, is wasting time?

kangax as reply to Diego comment, then Ajaxian as desert with my comments, but three of us completely in the wrong direction.

Why isNative?

As I though and as Diego Perini confirmed, sometimes we would like to trust the environment itself before we act in a certain way.
I wrote about an XMLHttpRequest wrapper for Internet Explorer, but cases are numerous, like a "not compliant" implementation of the Array.prototype.forEach, or others proto, or some injection problems about window.Array, window.String, or who knows what else.
Accordingly, since the JavaScript nature is dynamic, widely dynamic, we would like to be sure that a method is as "specs says" rather than how this or that library implemented it.

Can we trust an unreliable environment?

The first error we all committed was to find the "best way ever" to understand if a function/method has been changed, without considering the fact that the function isNative itself could be a fake!
No matter how good is our way to discover the native code because isNative(this, "isNative"); will ironically return false every time.
A way to partially solve the problem is the usage of const but this keyword makes sense only for FireFox and Safari, so Opera and even worse case Internet Explorer will not be able to create an immutable function.

/*@cc_on var // @*/ const
isNative = function(){
    // whatever You need
};

Above snippet, described in one of my old posts, will let us create an immutable function only for certain browsers, but the isNative problem, as I said, will persist for IE, Opera, and probably other browsers as well.

About proposed solutions, plus mine

Both kangax and Diego have written a couple of functions, the first one created a sandbox via iframe while the second one a string sniff.
Both solutions are not that perfect because of iframes or because a function toString method coul be changed arbitrary, since functions are objects as well.
My solution has probably some side effect as well, but unless smebody will be able to demonstrate that it could be hacked, which was for Diego case, I will consider my proposal the most reliable without iframes.

function isNative(object, method){
    var r = !!object && !/undefined|string/.test(typeof object[method]);
    if(r){
        var toString = (method = object[method]).toString;
        try{ // necessary for native functions (alert, as example)
            delete method.toString;
            r = method.toString !== Object.prototype.toString && /\[.+?\]/.test(method);
            if(method.toString != toString)
                method.toString = toString;
        }catch(e){
            r = true;
        }
    };
    return r;
};

Come on, break it and demonstrate that as I said, JavaScript is 100% unreliable programming language ;-)

P.S. this discussion remind me those days me and kentaromiura were trying to find a way to solve eval injections, we gave up after a week of tries hacking our code by our own :D