Make IOSink implement StringSink

tests/standalone/io/http_headers_state_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:isolate";
 import "dart:io";
 
 void test(int totalConnections, [String body]) {
   HttpServer.bind().then((server) {
 
     server.listen((HttpRequest request) {
       HttpResponse response = request.response;
       // Cannot mutate request headers.
       Expect.throws(() => request.headers.add("X-Request-Header", "value"),
                     (e) => e is HttpException);
       Expect.equals("value", request.headers.value("X-Request-Header"));
       request.listen((_) {}, onDone: () {
         // Can still mutate response headers as long as no data has been sent.
         response.headers.add("X-Response-Header", "value");
         if (body != null) {
-          response.addString(body);
+          response.write(body);
           // Cannot mutate response headers when data has been sent.
           Expect.throws(() => request.headers.add("X-Request-Header", "value2"),
                         (e) => e is HttpException);
         }
         response..close();
         // Cannot mutate response headers when data has been sent.
         Expect.throws(() => request.headers.add("X-Request-Header", "value3"),
                       (e) => e is HttpException);
       });
     });
 
     int count = 0;
     HttpClient client = new HttpClient();
     for (int i = 0; i < totalConnections; i++) {
       client.get("127.0.0.1", server.port, "/")
         .then((HttpClientRequest request) {
           if (body != null) {
             request.contentLength = -1;
           }
           // Can still mutate request headers as long as no data has been sent.
           request.headers.add("X-Request-Header", "value");
           if (body != null) {
-            request.addString(body);
+            request.write(body);
             // Cannot mutate request headers when data has been sent.
             Expect.throws(
                 () => request.headers.add("X-Request-Header", "value2"),
                 (e) => e is HttpException);
           }
           request.close();
           // Cannot mutate request headers when data has been sent.
           Expect.throws(() => request.headers.add("X-Request-Header", "value3"),
                         (e) => e is HttpException);
           return request.response;
@@ skipping 15 matching lines @@
         });
     }
 
   });
 }
 
 void main() {
   test(5);
   test(5, "Hello and goodbye");
 }