为什么在flutter中更新http包后不能在http.post方法中放入字符串

import 'package:http/http.dart' as http;

void postData() {
  String url = 'https://example.com/api';
  String body = 'Hello, World!';

  http.post(url, body: body).then((response) {
    print('Response status: ${response.statusCode}');
    print('Response body: ${response.body}');
  });
}

import 'package:http/http.dart' as http;

void postData() {
  String url = 'https://example.com/api';
  String body = 'Hello, World!';

  http.post(url, body: {'data': body}).then((response) {
    print('Response status: ${response.statusCode}');
    print('Response body: ${response.body}');
  });
}

import 'dart:convert';
import 'package:http/http.dart' as http;

void postData() {
  String url = 'https://example.com/api';
  Map<String, dynamic> body = {
    'name': 'John Doe',
    'age': 30,
  };

  http.post(url, body: jsonEncode(body)).then((response) {
    print('Response status: ${response.statusCode}');
    print('Response body: ${response.body}');
  });
}