Don't close existing connection on HttpServer close (and stream cancel).

tests/standalone/io/http_server_early_server_close_test.dart

-// Copyright (c) 2012, 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 "package:expect/expect.dart";
-import "dart:async";
-import "dart:io";
-import "dart:isolate";
-
-class Server {
-  static Future<int> start() {
-    return HttpServer.bind("127.0.0.1", 0).then((server) {
-      server.listen((HttpRequest request) {
-            Timer.run(server.close);
-          }, onError: (e) {
-            String msg = "No server errors expected: $e";
-            var trace = getAttachedStackTrace(e);
-            if (trace != null) msg += "\nStackTrace: $trace";
-            Expect.fail(msg);
-          });
-      return server.port;
-    });
-  }
-}
-
-class Client {
-  Client(int port) {
-    ReceivePort r = new ReceivePort();
-    HttpClient client = new HttpClient();
-    client.get("127.0.0.1", port, "/")
-        .then((HttpClientRequest request) {
-          return request.close();
-        })
-        .then((HttpClientResponse response) {
-          Expect.fail(
-              "Response should not be given, as not data was returned.");
-        })
-        .catchError((e) {
-          r.close();
-        });
-  }
-}
-
-main() {
-  Server.start().then((port) {
-    new Client(port);
-  });
-}