ScriptLoader = function(url, callAfter) {
  this.callAfterLoad = callAfter;
  var deferred = loadScript(url);
  deferred.addCallback(bind(this.scriptLoaded, this));
}

ScriptLoader.prototype.scriptLoaded = function(err) {
  if (err) {
    alert(err);
  }
  this.callAfterLoad();
}