こんなかんじですかねえ…。

lazyloader.js

(function (global) {
    var dosomething = function () {
        alert('do something');
    };

    var items = [], element,
        i, size, waitForLoad,
        interval = 100;
    items.push('https://www.google.co.jp/extern_chrome/dd37e4e7f3ce9237.js');
    items.push('https://www.google.co.jp/images/nav_logo117.png');
    size = waitForLoad = items.length;

    for (i = 0; i < size; i++) {
        if (items[i].substr(-3) === '.js') {
            element = global.document.createElement('script');
            element.type = 'text/javascript';
            element.src = items[i];
        } else if (items[i].substr(-4) === '.css') {
            element = global.document.createElement('link');
            element.type = 'text/css';
            element.rel = 'stylesheet';
            element.href = items[i];
        } else if (items[i].substr(-4) === '.png') {
            element = global.document.createElement('img');
            element.src = items[i];
        } else {
            throw new Error('^^;');
        }
        element.addEventListener('load', function (event) {
            --waitForLoad;
        });
        global.document.body.appendChild(element);
    }
    setTimeout(function () {
        if (waitForLoad === 0) {
            dosomething();
        } else {
            setTimeout(arguments.callee, interval);
        }
    }, interval);
})(this);