Keep track of when a socket has been destroyed

tests/standalone/io/socket_exception_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.
 //
 // Tests socket exceptions.
 
 import "package:expect/expect.dart";
 import "dart:async";
 import "dart:isolate";
 import "dart:io";
@@ skipping 100 matching lines @@
       Socket.connect("127.0.0.1", server.port).then((client) {
         client.listen((data) {}, onDone: server.close);
         client.destroy();
       });
     });
   }
 
   static void clientSocketAddDestroyNoErrorTest() {
     ServerSocket.bind("127.0.0.1", 0).then((server) {
       server.listen((socket) {
-        // Passive block data by not sobscribing to socket.
+        // Passive block data by not subscribing to socket.
       });
       Socket.connect("127.0.0.1", server.port).then((client) {
         client.listen((data) {}, onDone: server.close);
         client.add(new List.filled(1024 * 1024, 0));
         client.destroy();
       });
     });
   }
 
   static void clientSocketAddCloseNoErrorTest() {
@@ skipping 101 matching lines @@
     clientSocketAddCloseErrorTest();
     clientSocketAddCloseResultErrorTest();
     unknownHostTest();
   }
 }
 
 main() {
   SocketExceptionTest.testMain();
 }