code: function(name) {
      // Check the name ends with a /
      if (name.slice(-1) != "/") {
          name += "/"; // IE doesn't like substr(-1)
      }
  
      // Does this folder already exist?
      if (!this.files[name]) {
          fileAdd.call(this, name, null, {
              dir: true
          });
      }
      return this.files[name];
  }
docs: Add a (sub) folder in the current folder.
  @private
  @param {string} name the folder's name
  @return {Object} the new folder.