Make IOSink implement StringSink

tests/standalone/io/http_stream_close_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.
 //
 
 import "dart:io";
 import "dart:uri";
 
 main() {
   bool serverOnClosed = false;
@@ skipping 11 matching lines @@
     }
 
     server.listen((request) {
       request.listen(
           (_) {},
           onDone: () {
             request.response.done.then((_) {
               serverOnClosed = true;
               checkDone();
             });
-            request.response.addString("hello!");
+            request.response.write("hello!");
             request.response.close();
           });
       });
 
     client.postUrl(Uri.parse("http://127.0.0.1:${server.port}"))
         .then((request) {
           request.contentLength = "hello!".length;
           request.done.then((_) {
             clientOnClosed = true;
             checkDone();
           });
-          request.addString("hello!");
+          request.write("hello!");
           return request.close();
         })
         .then((response) {
           response.listen(
               (_) {},
               onDone: () {
                 requestOnClosed = true;
                 checkDone();
               });
         });
   });
 }