Make IOSink implement StringSink

tests/standalone/io/http_connection_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:async";
 import "dart:io";
 import "dart:uri";
 
 void testHttp10Close(bool closeRequest) {
   HttpServer.bind().then((server) {
     server.listen((request) {
       request.response.close();
     });
 
     Socket.connect("127.0.0.1", server.port)
       .then((socket) {
-        socket.addString("GET / HTTP/1.0\r\n\r\n");
+        socket.write("GET / HTTP/1.0\r\n\r\n");
         socket.listen(
           (data) {},
           onDone: () {
            if (!closeRequest) socket.destroy();
             server.close();
           });
         if (closeRequest) socket.close();
       });
   });
 }
 
 void testHttp11Close(bool closeRequest) {
   HttpServer.bind().then((server) {
     server.listen((request) {
      request.response.close();
     });
 
     Socket.connect("127.0.0.1", server.port)
       .then((socket) {
         List<int> buffer = new List<int>(1024);
-        socket.addString("GET / HTTP/1.1\r\nConnection: close\r\n\r\n");
+        socket.write("GET / HTTP/1.1\r\nConnection: close\r\n\r\n");
         socket.listen(
           (data) {},
           onDone: () {
             if (!closeRequest) socket.destroy();
             server.close();
           });
         if (closeRequest) socket.close();
       });
   });
 }
 
 void testStreamResponse() {
   HttpServer.bind().then((server) {
     server.listen((request) {
       // TODO(ajohnsen): Use timer (see old version).
       for (int i = 0; i < 10; i++) {
-        request.response.addString(
+        request.response.write(
             'data:${new DateTime.now().millisecondsSinceEpoch}\n\n');
       }
     });
 
     var client = new HttpClient();
     client.getUrl(Uri.parse("http://127.0.0.1:${server.port}"))
       .then((request) => request.close())
       .then((response) {
         int bytes = 0;
         response.listen(
@@ skipping 10 matching lines @@
   });
 }
 
 main() {
   testHttp10Close(false);
   testHttp10Close(true);
   testHttp11Close(false);
   testHttp11Close(true);
   testStreamResponse();
 }