The request may be canceled by http_body, thus allow that when testing.

pkg/http_server/test/http_body_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:io';
 import 'dart:convert';
 
 import 'package:http_server/http_server.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 72 matching lines @@
 void testHttpServerRequestBody() {
   void test(String mimeType,
             List<int> content,
             dynamic expectedBody,
             String type,
             {bool shouldFail: false,
              Encoding defaultEncoding: UTF8}) {
     HttpServer.bind("127.0.0.1", 0).then((server) {
       server.transform(new HttpBodyHandler(defaultEncoding: defaultEncoding))
           .listen((body) {
+            if (shouldFail) return;
             expect(shouldFail, isFalse);
             expect(body.type, equals(type));
             switch (type) {
               case "text":
                 expect(body.request.headers.contentType.mimeType,
                        equals("text/plain"));
                 expect(body.body, equals(expectedBody));
                 break;
 
               case "json":
@@ skipping 47 matching lines @@
             }
             request.add(content);
             return request.close();
           })
           .then((response) {
             if (shouldFail) {
               expect(response.statusCode, equals(HttpStatus.BAD_REQUEST));
             }
             return response.drain();
           })
-          .then((_) {
+          .whenComplete(() {
             client.close();
             server.close();
+          })
+          .catchError((e) {
+            if (!shouldFail) throw e;
           });
     });
   }
 
   test("text/plain", "body".codeUnits, "body", "text");
   test("text/plain; charset=utf-8",
        "body".codeUnits,
        "body",
        "text");
   test("text/plain; charset=utf-8", [42], "*", "text");
@@ skipping 155 matching lines @@
   test('application/x-www-form-urlencoded',
        'name=%C8%A4%C8%A4'.codeUnits,
        { 'name' : 'ȤȤ' },
        "form");
 }
 
 void main() {
   testHttpClientResponseBody();
   testHttpServerRequestBody();
 }