Spy on requests through MM and call a callback with ms inflight

spy.js

function spy(layer, callback) {
    var inflight = {};
    var rt = layer.requestManager.requestTile;
    layer.requestManager.requestTile = function() {
        inflight[arguments[0]] = +new Date();
        rt.apply(layer.requestManager, arguments);
    };
    layer.requestManager.addCallback('requestcomplete', function(s, img) {
        callback(+new Date() - inflight[img.id]);
        delete inflight[img.id];
    });
}