Add writeString method to OutputStream

tests/standalone/src/io/HttpShutdownTest.dart

 // Copyright (c) 2012, 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 test1(int totalConnections) {
   // Server which just closes immediately.
@@ skipping 19 matching lines @@
     };
   }
 }
 
 
 void test2(int totalConnections) {
   // Server which responds without waiting for request body.
   HttpServer server = new HttpServer();
   server.listen("127.0.0.1", 0, totalConnections);
   server.onRequest = (HttpRequest request, HttpResponse response) {
-    response.writeString("!dlrow ,olleH");
+    response.outputStream.writeString("!dlrow ,olleH");
     response.outputStream.close();
   };
 
   int count = 0;
   HttpClient client = new HttpClient();
   for (int i = 0; i < totalConnections; i++) {
     HttpClientConnection conn = client.get("127.0.0.1", server.port, "/");
     conn.onRequest = (HttpClientRequest request) {
-      request.writeString("Hello, world!");
+      request.outputStream.writeString("Hello, world!");
       request.outputStream.close();
     };
     conn.onResponse = (HttpClientResponse response) {
       count++;
       if (count == totalConnections) {
         client.shutdown();
         server.close();
       }
     };
   }
 }
 
 
 void test3(int totalConnections) {
   // Server which responds when request body has been received.
   HttpServer server = new HttpServer();
   server.listen("127.0.0.1", 0, totalConnections);
   server.onRequest = (HttpRequest request, HttpResponse response) {
     request.inputStream.onClosed = () {
-      response.writeString("!dlrow ,olleH");
+      response.outputStream.writeString("!dlrow ,olleH");
       response.outputStream.close();
     };
   };
 
   int count = 0;
   HttpClient client = new HttpClient();
   for (int i = 0; i < totalConnections; i++) {
     HttpClientConnection conn = client.get("127.0.0.1", server.port, "/");
     conn.onRequest = (HttpClientRequest request) {
-      request.writeString("Hello, world!");
+      request.outputStream.writeString("Hello, world!");
       request.outputStream.close();
     };
     conn.onResponse = (HttpClientResponse response) {
       count++;
       if (count == totalConnections) {
         client.shutdown();
         server.close();
       }
     };
   }
 }
 
 
 void main() {
   test1(1);
   test1(10);
   test2(1);
   test2(10);
   test3(1);
   test3(10);
 }