带有 Dart 的 CORS，我如何让它工作

刚刚开始修改DART，我决定编写一个简单的http服务器和一个客户端.我的服务器代码:

#import("dart:io");

final HOST = "127.0.0.1";
final PORT = 8080;
final LOG_REQUESTS = true;

void main() {
  HttpServer server = new HttpServer();
  server.addRequestHandler((HttpRequest request) => true, requestReceivedHandler);
  server.listen(HOST, PORT);
  print("Server is running on ${PORT}."); 
}

void requestReceivedHandler(HttpRequest request, HttpResponse response) {
  var pathname = request.uri;
  var apiresponse="";
  if (LOG_REQUESTS) {
    print("Request: ${request.method} ${pathname}");
  }
  if(pathname == '/api'){
    response.headers.set(HttpHeaders.CONTENT_TYPE, "text/plain; charset=UTF-8");
    response.headers.add("Access-Control-Allow-Methods", "POST, OPTIONS, GET");
    response.headers.add("Access-Control-Allow-Origin", "*");
    response.headers.add('Access-Control-Allow-Headers', '*');
    print('welcome to the good life');
    response.outputStream.writeString("API Call");
    response.outputStream.close();
  }
}

我的客户代码:

#import('dart:html');
#import('dart:json');

class dartjson {

  dartjson() {
  }

  void run() {
    write("Hello World!");
  }




  void fetchFeed(){
    XMLHttpRequest xhr = new XMLHttpRequest();
    var url = "http://127.0.0.1:8080/api";
    xhr.open("GET", url, true);
    xhr.setRequestHeader('Content-Type', 'text/plain');
    //xhr.setRequestHeader('Access-Control-Request-Headers', 'http://127.0.0.1:3030');
    xhr.send();
    print(xhr.responseText);
    document.query('#status').innerHTML = xhr.responseText;

  }



void main() {
  new dartjson().fetchFeed();
}

我不断发现错误:

XMLHttpRequest cannot load http://127.0.0.1:8080/api. Origin
http://127.0.0.1:3030 is not allowed by Access-Control-Allow-Origin.

我做错了什么？