WorkerGlobalScope.fetch doesn't serialize TypedArrays properly

[jbrownsw]

• Dart SDK version: 2.16.2
• Platform: Windows 11
• Browser: Chrome 100.0.4896.127

I was attempting to implement a fetch based transport for the grpc-dart package as opposed to xhr due to the following issue:
grpc/grpc-dart#511

My initial implementation was calling WorkerGlobalScope.fetch, but I could never get the body right. I have a Uint8List that I wanted in the body which should just map to a javascript Uint8Array, but instead it would convert it to just a comma separated string of ints. I suspect it has to do with convertDartToNative_Dictionary which fetch calls internally.
https://github.com/jbrownsw/grpc-dart/blob/26b5dbb8dbbaccea04bca7cc866a4035a1b503d3/lib/src/client/transport/fetch_transport.dart

My workaround was just to define the interop init type here and instead of calling wgs.fetch I use callMethod:
jbrownsw/grpc-dart@e627db5

A minimal(ish) sample would be something like the following:

// ignore_for_file: avoid_web_libraries_in_flutter
import 'dart:html';
import 'dart:typed_data';
import 'dart:js_util' as js_util;

void main() async {
  final data = Uint8List.fromList([
    // Don't format me
    0, 0, 0, 0, 65, 10, 59, 81, 117, 103, 110, 87, 117, 50, 67, 49, 114, 50,
    73, 88, 57, 107, 54, 56, 118, 106, 65, 73, 65, 58, 52, 100, 101, 48, 55,
    48, 55, 50, 45, 97, 51, 54, 54, 45, 52, 49, 51, 48, 45, 98, 101, 98, 54,
    45, 97, 55, 50, 55, 98, 101, 55, 55, 51, 100, 102, 54, 16, 128, 130, 104
  ]);
  final headers = <String, String>{
    'Content-Type': 'application/grpc-web+proto',
    'X-User-Agent': 'grpc-web-dart/0.1',
    'X-Grpc-Web': '1',
  };

  final init = <String, dynamic>{
    'cache': 'default',
    'credentials': 'omit',
    'integrity': '',
    'keepalive': true,
    'method': 'POST',
    'mode': 'cors',
    'redirect': 'follow',
    'referrerPolicy': 'origin',
    'body': data,
    'headers': js_util.jsify(headers),
    'data': data,
  };

  final wgs = WorkerGlobalScope.instance;
  final response = await wgs.fetch('https://reqbin.com/echo/post/json', init);
}

I'm then using chrome dev tools to monitor the network traffic and I'm expecting to see a 70 byte message being sent. However, instead I see a content-length 222 message because it converts data to a comma separated list of ints (essentially just toString()).