code: function(callback) {
      var packages = Object.keys(this.packageToVersions);
      async.parallel(packages.map(function(package) {
        var packageRootUrl = url.resolve(this.master, package);
        return this.downloadPackageRootObject.bind(this, package, packageRootUrl);
      }.bind(this)), function(err) {
        return callback && callback(err);
      });
    }
docs: Download a package root object for each package.
  
  @param {Function} callback invoke when done.