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

tests/standalone/io/http_10_test.dart

 // (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 12 matching lines @@
           Expect.equals(-1, request.contentLength);
           var response = request.response;
           response.contentLength = 1;
           Expect.equals("1.0", request.protocolVersion);
           response.done
               .then((_) => Expect.fail("Unexpected response completion"))
               .catchError((error) => Expect.isTrue(error is HttpException));
           response.write("Z");
           response.write("Z");
           response.close();
-          Expect.throws(() => response.write("x"),
-                        (e) => e is StateError);
+          response.write("x");
         },
         onError: (e) {
           String msg = "Unexpected error $e";
           var trace = getAttachedStackTrace(e);
           if (trace != null) msg += "\nStackTrace: $trace";
           Expect.fail(msg);
         });
 
     int count = 0;
     makeRequest() {
@@ skipping 180 matching lines @@
   });
 }
 
 
 void main() {
   testHttp10NoKeepAlive();
   testHttp10ServerClose();
   testHttp10KeepAlive();
   testHttp10KeepAliveServerCloses();
 }