The source code

    /*!
     * Ext JS Library 3.2.1
     * Copyright(c) 2006-2010 Ext JS, Inc.
     * licensing@extjs.com
     * http://www.extjs.com/license
     */
    /**
     * @class Ext.Loader
     * @singleton
     * Simple class to help load JavaScript files on demand
     */
    Ext.Loader = Ext.apply({}, {
        /**
         * Loads a given set of .js files. Calls the callback function when all files have been loaded
         * Set preserveOrder to true to ensure non-parallel loading of files if load order is important
         * @param {Array} fileList Array of all files to load
         * @param {Function} callback Callback to call after all files have been loaded
         * @param {Object} scope The scope to call the callback in
         * @param {Boolean} preserveOrder True to make files load in serial, one after the other (defaults to false)
         */
        load: function(fileList, callback, scope, preserveOrder) {
            var scope       = scope || this,
                head        = document.getElementsByTagName("head")[0],
                fragment    = document.createDocumentFragment(),
                numFiles    = fileList.length,
                loadedFiles = 0,
                me          = this;

            /**
             * Loads a particular file from the fileList by index. This is used when preserving order
             */
            var loadFileIndex = function(index) {
                head.appendChild(
                    me.buildScriptTag(fileList[index], onFileLoaded)
                );
            };

            /**
             * Callback function which is called after each file has been loaded. This calls the callback
             * passed to load once the final file in the fileList has been loaded
             */
            var onFileLoaded = function() {
                loadedFiles ++;

                //if this was the last file, call the callback, otherwise load the next file
                if (numFiles == loadedFiles && typeof callback == 'function') {
                    callback.call(scope);
                } else {
                    if (preserveOrder === true) {
                        loadFileIndex(loadedFiles);
                    }
                }
            };

            if (preserveOrder === true) {
                loadFileIndex.call(this, 0);
            } else {
                //load each file (most browsers will do this in parallel)
                Ext.each(fileList, function(file, index) {
                    fragment.appendChild(
                        this.buildScriptTag(file, onFileLoaded)
                    );
                }, this);

                head.appendChild(fragment);
            }
        },

        /**
         * @private
         * Creates and returns a script tag, but does not place it into the document. If a callback function
         * is passed, this is called when the script has been loaded
         * @param {String} filename The name of the file to create a script tag for
         * @param {Function} callback Optional callback, which is called when the script has been loaded
         * @return {Element} The new script ta
         */
        buildScriptTag: function(filename, callback) {
            var script  = document.createElement('script');
            script.type = "text/javascript";
            script.src  = filename;

            //IE has a different way of handling