myshelf/lib/handlers/echo_handler.dart

import 'dart:convert';
import 'package:shelf/shelf.dart';

Future<Response> handleEcho(Request request) async {
  final payload = await request.readAsString();
  final data = jsonDecode(payload) as Map;
  return Response.ok(
    jsonEncode(data),
    headers: {'Content-Type': 'application/json'},
  );
}