Don't throw exceptions when adding to a IOSink(StreamSink) after close.

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 77 matching lines @@
         Expect.isFalse(exceptionCaught);
         Expect.isFalse(wrongExceptionCaught);
         try {
           List<int> buffer = new List<int>(10);
           client.add(buffer);
         } on StateError catch (ex) {
           exceptionCaught = true;
         } catch (ex) {
           wrongExceptionCaught = true;
         }
-        Expect.isTrue(exceptionCaught);
+        Expect.isFalse(exceptionCaught);
         Expect.isFalse(wrongExceptionCaught);
 
         server.close();
       });
     });
   }
 
   static void clientSocketDestroyNoErrorTest() {
     ServerSocket.bind("127.0.0.1", 0).then((server) {
       server.listen((socket) {
@@ skipping 124 matching lines @@
     clientSocketAddCloseErrorTest();
     clientSocketAddCloseResultErrorTest();
     unknownHostTest();
   }
 }
 
 main() {
   SocketExceptionTest.testMain();
 }