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

tests/standalone/io/http_content_length_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 22 matching lines @@
             });
           // write with content length 0 closes the connection and
           // reports an error.
           response.write("x");
           // Subsequent write are ignored as there is already an
           // error.
           response.write("x");
           // After an explicit close, write becomes a state error
           // because we have said we will not add more.
           response.close();
-          Expect.throws(() => response.write("x"),
-                        (e) => e is StateError);
+          response.write("x");
         },
         onError: (e) {
           String msg = "Unexpected server error $e";
           var trace = getAttachedStackTrace(e);
           if (trace != null) msg += "\nStackTrace: $trace";
           Expect.fail(msg);
         });
 
     HttpClient client = new HttpClient();
     for (int i = 0; i < totalConnections; i++) {
@@ skipping 44 matching lines @@
                     .then((_) {
                       Expect.fail("Unexpected successful response completion");
                     })
                     .catchError((error) {
                       Expect.isTrue(error is HttpException, "[$error]");
                       if (++serverCount == totalConnections) {
                         server.close();
                       }
                     });
                 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 clientCount = 0;
@@ skipping 48 matching lines @@
           request.listen(
               (d) {},
               onDone: () {
                 response.write("x");
                 Expect.throws(
                     () => response.headers.chunkedTransferEncoding = false,
                     (e) => e is HttpException);
                 response.write("x");
                 response.write("x");
                 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;
@@ skipping 69 matching lines @@
   testNoBody(5, false);
   testNoBody(25, false);
   testNoBody(5, true);
   testNoBody(25, true);
   testBody(5, false);
   testBody(5, true);
   testBodyChunked(5, false);
   testBodyChunked(5, true);
   testSetContentLength();
 }