Make IOSink implement StringSink

tests/standalone/io/http_basic_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 "dart:isolate";
@@ skipping 92 matching lines @@
     Expect.equals("POST", request.method);
     response.contentLength = request.contentLength;
     request.pipe(response);
   }
 
   // Echo the request content back to the response.
   void _zeroToTenHandler(HttpRequest request) {
     var response = request.response;
     Expect.equals("GET", request.method);
     request.listen((_) {}, onDone: () {
-        response.addString("01234567890");
+        response.write("01234567890");
         response.close();
       });
   }
 
   // Return a 404.
   void _notFoundHandler(HttpRequest request) {
     var response = request.response;
     response.statusCode = HttpStatus.NOT_FOUND;
     response.headers.set("Content-Type", "text/html; charset=UTF-8");
-    response.addString("Page not found");
+    response.write("Page not found");
     response.close();
   }
 
   // Return a 301 with a custom reason phrase.
   void _reasonForMovingHandler(HttpRequest request) {
     var response = request.response;
     response.statusCode = HttpStatus.MOVED_PERMANENTLY;
     response.reasonPhrase = "Don't come looking here any more";
     response.close();
   }
@@ skipping 87 matching lines @@
 
   TestServerMain testServerMain = new TestServerMain();
 
   void runTest(int port) {
     int count = 0;
     HttpClient httpClient = new HttpClient();
     void sendRequest() {
       httpClient.post("127.0.0.1", port, "/echo")
           .then((request) {
             if (chunkedEncoding) {
-              request.addString(data.substring(0, 10));
-              request.addString(data.substring(10, data.length));
+              request.write(data.substring(0, 10));
+              request.write(data.substring(10, data.length));
             } else {
               request.contentLength = data.length;
-              request.addString(data);
+              request.write(data);
             }
             return request.close();
           })
           .then((response) {
             Expect.equals(HttpStatus.OK, response.statusCode);
             StringBuffer body = new StringBuffer();
             response.listen(
               (data) => body.add(new String.fromCharCodes(data)),
               onDone: () {
                 Expect.equals(data, body.toString());
@@ skipping 64 matching lines @@
 }
 
 void main() {
   testStartStop();
   testGET();
   testPOST(true);
   testPOST(false);
   test404();
   testReasonPhrase();
 }