/**
 * @constructor
 */
webfont.FontApiUrlBuilder = function(apiUrl) {
  if (apiUrl) {
    this.apiUrl_ = apiUrl;
  } else {
    var protocol = 'https:' == window.location.protocol ? 'https:' : 'http:';

    this.apiUrl_ = protocol + webfont.FontApiUrlBuilder.DEFAULT_API_URL;
  }  
  this.fontFamilies_ = null;
};

webfont.FontApiUrlBuilder.DEFAULT_API_URL = '//fonts.googleapis.com/css';

webfont.FontApiUrlBuilder.prototype.setFontFamilies = function(fontFamilies) {
  // maybe clone?
  this.fontFamilies_ = fontFamilies;
};

webfont.FontApiUrlBuilder.prototype.webSafe = function(string) {
  return string.replace(/ /g, '+');
};

webfont.FontApiUrlBuilder.prototype.build = function() {
  if (!this.fontFamilies_) {
    throw new Error('No fonts to load !');
  }
  var length = this.fontFamilies_.length;
  var sb = [];

  for (var i = 0; i < length; i++) {
    sb.push(this.webSafe(this.fontFamilies_[i]));
  }
  var url = this.apiUrl_ + '?family=' + sb.join('%7C'); // '|' escaped.

  return url;
};

/**
 * @constructor
 */
webfont.FontApiUrlBuilder = function(apiUrl) {
  if (apiUrl) {
    this.apiUrl_ = apiUrl;
  } else {
    var protocol = 'https:' == window.location.protocol ? 'https:' : 'http:';

    this.apiUrl_ = protocol + webfont.FontApiUrlBuilder.DEFAULT_API_URL;
  }  
  this.fontFamilies_ = null;
};

webfont.FontApiUrlBuilder.DEFAULT_API_URL = '//fonts.googleapis.com/css';

webfont.FontApiUrlBuilder.prototype.setFontFamilies = function(fontFamilies) {
  // maybe clone?
  this.fontFamilies_ = fontFamilies;
};

webfont.FontApiUrlBuilder.prototype.webSafe = function(string) {
  return string.replace(/ /g, '+');
};

webfont.FontApiUrlBuilder.prototype.build = function() {
  if (!this.fontFamilies_) {
    throw new Error('No fonts to load !');
  }
  if (this.apiUrl_.indexOf("kit=") != -1) {
    return this.apiUrl_;
  }
  var length = this.fontFamilies_.length;
  var sb = [];

  for (var i = 0; i < length; i++) {
    sb.push(this.webSafe(this.fontFamilies_[i]));
  }
  var url = this.apiUrl_ + '?family=' + sb.join('%7C'); // '|' escaped.

  return url;
};

Change the url builder to handle integrations.