code: function ()
    {
      /** @type {Object.<string, ModuleDef>} */
      var modules = {};
      ((typeof this.options.externalModules === 'string' ?
        [this.options.externalModules] : this.options.externalModules
        ) || []).
        concat (this.options.builtinModules).
        forEach (function (moduleName)
      {
        // Ignore redundant names.
        if (!modules[moduleName]) {
          /** @type {ModuleDef} */
          var module = modules[moduleName] = new ModuleDef (moduleName);
          module.external = true;
        }
      });
      return modules;
    }
docs: Registers the configured external modules so that they can be ignored during the build output generation.
  @returns {Object.<string, ModuleDef>}
  @private