anchor.diff:     * Validate that the datum is a list of strings, and return it.
      */
     List<String> asStringList() {
  -    if (!isStringList()) {
  +    if (!isStringList) {
         throw new RequestFailure(new Response.invalidParameter(request, path,
             "be a list of strings"));
       }
  
positive.code:    * Validate that the datum is a map from strings to strings, and return it.
     */
    Map<String, String> asStringMap() {
      if (!isStringMap()) {
        throw new RequestFailure(new Response.invalidParameter(request, path,
            "be a string map"));
      }
  
negative.code:   /**
     * Determine if the datum is a list of strings.
     */
    bool get isStringList {
      if (datum is! List) {
        return false;
      }
  