Make IOSink implement StringSink

tests/standalone/io/http_server_response_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:async";
@@ skipping 69 matching lines @@
           response.close();
           response.done.then((_) => server.close());
         });
     controller.close();
   }, bytes: 0);
 }
 
 void testBadResponseAdd() {
   testServerRequest((server, request) {
     request.response.contentLength = 0;
-    request.response.add([0]);
+    request.response.writeBytes([0]);
     request.response.done.catchError((error) {
       server.close();
     }, test: (e) => e is HttpException);
   });
 
   testServerRequest((server, request) {
     request.response.contentLength = 5;
-    request.response.add([0, 0, 0]);
-    request.response.add([0, 0, 0]);
+    request.response.writeBytes([0, 0, 0]);
+    request.response.writeBytes([0, 0, 0]);
     request.response.done.catchError((error) {
       server.close();
     }, test: (e) => e is HttpException);
   });
 
   testServerRequest((server, request) {
     request.response.contentLength = 0;
-    request.response.add(new Uint8List(64 * 1024));
-    request.response.add(new Uint8List(64 * 1024));
-    request.response.add(new Uint8List(64 * 1024));
+    request.response.writeBytes(new Uint8List(64 * 1024));
+    request.response.writeBytes(new Uint8List(64 * 1024));
+    request.response.writeBytes(new Uint8List(64 * 1024));
     request.response.done.catchError((error) {
       server.close();
     }, test: (e) => e is HttpException);
   });
 }
 
 void testBadResponseClose() {
   testServerRequest((server, request) {
     request.response.contentLength = 5;
     request.response.close();
     request.response.done.catchError((error) {
       server.close();
     }, test: (e) => e is HttpException);
   });
 
   testServerRequest((server, request) {
     request.response.contentLength = 5;
-    request.response.add([0]);
+    request.response.writeBytes([0]);
     request.response.close();
     request.response.done.catchError((error) {
       server.close();
     }, test: (e) => e is HttpException);
   });
 }
 
 void main() {
   testResponseDone();
   testResponseAddStream();
   testBadResponseAdd();
   testBadResponseClose();
 }