code: function repeatDirective(items, keyFnOrTemplate, template) {
    if (template === undefined) {
      template = keyFnOrTemplate;
    }
  
    return function(part) {
      part.setValue(items.map((item, index) => template(item, index)));
    };
  }
docs: Loop through 'items' and call 'template'.
  No concept of efficient re-ordering possible in server context,
  so this is a simple no-op map operation.
  
  @param { Array<any> } items
  @param { function } keyFnOrTemplate
  @param { (item: any, index: number) => TemplateResult } [template]
  @returns { (part: Part) => void }