harvestは根本的な不具合があったので、ローダーだけクラスにしてみた

assets.js

(function(win, doc){

	var Assets = function(){
		this.items = [];
		this.nodes = [];
		this.attrs = {
			callback: null,
			append: true
		};
	};

	(function(){

		this.supportAsync = doc.createElement("script").async !== undefined;
		this.head = doc.getElementsByTagName("head")[0];

		this.items = null;
		this.nodes = null;
		this.count = 0;
		this.callback = null;
		attrs: null,

		this.set = function(key, value){
			this.attrs[key] = value;
		};

		this.add = function(files){
			var my = this;
			this._each(files, function(value){
				my.items.push(value);
			});
			this.count = this.items.length;
		};

		this.load = function(){
			var my = this;
			this._each(this.items, function(url){
				var el = doc.createElement("script");
				my._on(el, "load", my._bind(my._process, my, el));
				my._on(el, "readystatechange", my._bind(my._process, my, el));
				el.type = "text/javascript";
				el.src = url;
				if(my.supportAsync){
					el.async = false;
					my.head.appendChild(el);
				}
				my.nodes.push(el);
			});
		};

		this.append = function(){
			var my = this;
			if(! this.supportAsync){
				this._each(this.nodes, function(el){
					my.head.appendChild(el);
				});
			}
		};

		this._process = function(e, el){
			if(el._loaded){ return; }
			if(e.type === "load" || /^(complete|loaded)$/.test(el.readyState)){
				el._loaded = true;
				if(! -- this.count){ this._complete(); }
			}
		};

		this._complete = function(){
			var my = this;
			if(! this.supportAsync && this.attrs.append){
				this._each(this.nodes, function(el){
					my.head.appendChild(el);
				});
			}
			if(typeof this.attrs.callback === "function"){
				this.attrs.callback(this);
			}
		};

		this._each = function(o, callback){
			for(var i in o){
				if(! o.hasOwnProperty(i)){ continue; }
				if(false === callback(o[i], i, o)){ break; }
			}
		};

		this._on = function(el, name, callback){
			if(el.addEventListener){ return el.addEventListener(name, callback, false); }
			if(el.attachEvent){ return el.attachEvent("on" + name, callback); }
		};

		this._bind = function(func, obj, arg){
			return function(){
				var args = [].slice.call(arguments);
				args.push(arg);
				func.apply(obj, args);
			};
		};

	}).call(Assets.prototype);

	win.Assets = Assets;

}(window, document));