function load(url, successCallback, errorCallback) {
  var xhr;
  xhr = window.ActiveXObject ? new window.ActiveXObject('Microsoft.XMLHTTP')
                             : new XMLHttpRequest();

  // async, however scripts will be executed in the order they are in the
  // DOM to mirror normal script loading.
  xhr.open('GET', url, true);
  if ('overrideMimeType' in xhr) {
    xhr.overrideMimeType('text/plain');
  }
  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4) {
      if (xhr.status === 0 || xhr.status === 200) {
        successCallback(xhr.responseText);
      } else {
        errorCallback();
        throw new Error('Could not load ' + url);
      }
    }
  };
  return xhr.send(null);
}
Load script from the provided url and pass the content to the callback.

@param {string} url The location of the script src
@param {function} callback Function to call with the content of url
@internal