zones.js mangles XMLHttpRequest constants

[benweissmann]

the patchClass() function doesn't copy over constants from the patched class. This means that a patched XMLHttpRequest won't have the constants XMLHttpRequest.DONE, etc. This breaks the common pattern of:

var req = new XMLHttpRequest();
req.onreadystatechange = function() {
    if (req.readyState === XMLHttpRequest.DONE) {
        // do something with the result
    }
};

patchClass() should copy over static properties of a class.