解析器，则可以在客户端上使用 File API 来阅读和上传文件内容作为 dataUrl 。此API在浏览器中提供了相当好的支持（请参阅 http://caniuse.com/filereader ）。

Until a multipart encoding parser is available, you can use File API on client to read and upload file content as dataUrl. This API has quite good support in browsers ( see http://caniuse.com/filereader ).

在客户端：

import 'dart:html';

main() {
  InputElement uploadInput = query('#upload');
  uploadInput.onChange.listen((e) {
    // read file content as dataURL
    final files = uploadInput.files;
    if (files.length == 1) {
      final file = files[0];
      final reader = new FileReader();
      reader.onLoad.listen((e) {
        sendDatas(reader.result);
      });
      reader.readAsDataUrl(file);
    }
  });
}

/// send data to server
sendDatas(dynamic data) {
  final req = new HttpRequest();
  req.onReadyStateChange.listen((Event e) {
    if (req.readyState == HttpRequest.DONE &&
        (req.status == 200 || req.status == 0)) {
      window.alert("upload complete");
    }
  });
  req.open("POST", "http://127.0.0.1:8080/upload");
  req.send(data);
}

而在服务器端：

import 'dart:io';

main() {
  final server = new HttpServer();
  server.listen('127.0.0.1', 8080);
  server.addRequestHandler((request) => request.path == '/upload' 
      && request.method.toLowerCase() == 'post'
      , (HttpRequest request, HttpResponse response) {
    _readBody(request, (body) {

      // handle your dataURL
      // example with image : data:image/jpeg;base64,/9j/4S2YRXhpZgAATU0AK... 

      // return result
      response.statusCode = HttpStatus.CREATED;
      response.contentLength = 0;
      response.outputStream.close();
    });
  });
}

/// Read body of [request] and call [handleBody] when complete.
_readBody(HttpRequest request, void handleBody(String body)) {
  String bodyString = ""; // request body byte data
  final completer = new Completer();
  final sis = new StringInputStream(request.inputStream, Encoding.UTF_8);
  sis.onData = (){
    bodyString = bodyString.concat(sis.read());
  };
  sis.onClosed = () {
    completer.complete("");
  };
  sis.onError = (Exception e) {
    print('exeption occured : ${e.toString()}');
  };
  // process the request and send a response
  completer.future.then((_){
    handleBody(bodyString);
  });
}

参考文献：

• 使用File API读取JavaScript中的文件

• Reading files in JavaScript using the File APIs