fix(http_client_conformance_tests): multipart server (#1292)

pkgs/http_client_conformance_tests/lib/src/multipart_server.dart

@@ -23,22 +23,27 @@ void hybridMain(StreamChannel<Object?> channel) async {
   server = (await HttpServer.bind('localhost', 0))
     ..listen((request) async {
       request.response.headers.set('Access-Control-Allow-Origin', '*');
+      request.response
+        ..contentLength = 0
+        ..statusCode = HttpStatus.ok;
+
       if (request.method == 'OPTIONS') {
         // Handle a CORS preflight request:
         // https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#preflighted_requests
         request.response.headers
           ..set('Access-Control-Allow-Methods', '*')
           ..set('Access-Control-Allow-Headers', '*');
+        await request.response.close();
       } else {
         final headers = <String, List<String>>{};
         request.headers.forEach((field, value) {
           headers[field] = value;
         });
         final body =
             await const Utf8Decoder().bind(request).fold('', (x, y) => '$x$y');
-        channel.sink.add((headers, body));
+        await request.response.close();
+        channel.sink.add([headers, body]);
       }
-      unawaited(request.response.close());
     });
 
   channel.sink.add(server.port);

pkgs/http_client_conformance_tests/lib/src/multipart_tests.dart

@@ -34,8 +34,9 @@ void testMultipartRequests(Client client,
       request.files.add(MultipartFile.fromString('file1', 'Hello World'));
 
       await client.send(request);
-      final (headers, body) =
-          await httpServerQueue.next as (Map<String, List<String>>, String);
+      final serverRequest = await httpServerQueue.next as List;
+      final headers = (serverRequest[0] as Map).cast<String, List<Object?>>();
+      final body = serverRequest[1] as String;
       expect(headers['content-length']!.single, '${request.contentLength}');
       expect(headers['content-type']!.single,
           startsWith('multipart/form-data; boundary='));