Fix raw_server_socket_cancel_test on macos by always call close on done.

tests/standalone/io/raw_server_socket_cancel_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";
 import "dart:async";
 import "dart:io";
 import "dart:isolate";
 
 void testCancelResubscribeServerSocket(int socketCount, int backlog) {
   var acceptCount = 0;
   var doneCount = 0;
   var errorCount = 0;
   var earlyErrorCount = 0;
 
   ReceivePort port = new ReceivePort();
 
   RawServerSocket.bind("127.0.0.1", 0, backlog: backlog).then((server) {
     Expect.isTrue(server.port > 0);
 
     void checkDone() {
       if (doneCount + errorCount + earlyErrorCount == socketCount) {
         port.close();
+        // Be sure to close as subscription.cancel may not be called, if
+        // backlog prevents acceptCount to grow to socketCount / 2.
+        socket.close();
       }
     }
 
     var subscription;
     subscription = server.listen((client) {
       client.writeEventsEnabled = false;
       client.listen((event) {
         switch(event) {
           case RawSocketEvent.READ:
             client.read();
@@ skipping 61 matching lines @@
         checkDone();
       });
     }
   });
 }
 
 void main() {
   testCancelResubscribeServerSocket(10, 20);
   testCancelResubscribeServerSocket(20, 5);
 }