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

tests/standalone/io/http_detach_socket_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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 import "package:expect/expect.dart";
@@ skipping 44 matching lines @@
       response.contentLength = 0;
       response.close();
       Expect.throws(response.detachSocket);
       server.close();
     });
 
     Socket.connect("127.0.0.1", server.port).then((socket) {
       socket.write("GET / HTTP/1.1\r\n"
                    "content-length: 0\r\n\r\n");
       socket.listen((_) {}, onDone: () {
-          socket.close();
-        });
+        socket.close();
+      });
     });
   });
 }
 
 void testClientDetachSocket() {
   ServerSocket.bind("127.0.0.1", 0).then((server) {
     server.listen((socket) {
       socket.write("HTTP/1.1 200 OK\r\n"
                        "\r\n"
                        "Test!");
@@ skipping 34 matching lines @@
         });
       });
   });
 }
 
 void main() {
   testServerDetachSocket();
   testBadServerDetachSocket();
   testClientDetachSocket();
 }