HttpClientRequest.contentLength incorrectly transmitted when using contentLength and header

[donny-dont]

This is a simple reproduction of the issue dart-lang/http#110 is meant to fix

Given the following code

Future post() async {
  Map jsonData = {
    'name': 'Han Solo',
    'job': 'reluctant hero',
    'BFF': 'Chewbacca',
    'ship': 'Millennium Falcon',
    'weakness': 'smuggling debts'
  };

  var request = await new HttpClient()
      .openUrl('POST', Uri.parse('https://httpbin.org/post'));

  final encoded = JSON.encode(jsonData);
  request.contentLength = encoded.length;
  request.headers.set('content-length', encoded.length);
  request.write(encoded);

  HttpClientResponse response = await request.close();
  await for (var contents in response.transform(UTF8.decoder)) {
    print(contents);
  }
}

The server returns

{
  "args": {}, 
  "data": "", 
  "files": {}, 
  "form": {}, 
  "headers": {
    "Accept-Encoding": "gzip", 
    "Connection": "close", 
    "Content-Length": "0", 
    "Host": "httpbin.org", 
    "User-Agent": "Dart/2.0 (dart:io)"
  }, 
  "json": null, 
  "origin": "173.230.196.25", 
  "url": "https://httpbin.org/post"
}

However if request.contentLength is not set or if the call to request.headers.set('content-length', encoded.length) is moved above the setter the following response is received from the server.

{
  "args": {}, 
  "data": "{\"name\":\"Han Solo\",\"job\":\"reluctant hero\",\"BFF\":\"Chewbacca\",\"ship\":\"Millennium Falcon\",\"weakness\":\"smuggling debts\"}", 
  "files": {}, 
  "form": {}, 
  "headers": {
    "Accept-Encoding": "gzip", 
    "Connection": "close", 
    "Content-Length": "116", 
    "Host": "httpbin.org", 
    "User-Agent": "Dart/2.0 (dart:io)"
  }, 
  "json": {
    "BFF": "Chewbacca", 
    "job": "reluctant hero", 
    "name": "Han Solo", 
    "ship": "Millennium Falcon", 
    "weakness": "smuggling debts"
  }, 
  "origin": "173.230.196.25", 
  "url": "https://httpbin.org/post"
}

[robinloxley1]

It happens on 0.11.3+16 as well.
The workaround is to let library to decide content-length, without setting it to headers.