Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/standalone/io/http_proxy_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";
 
 class Server {
   HttpServer server;
@@ skipping 25 matching lines @@
           Expect.equals(1, request.headers[HttpHeaders.VIA].length);
           Expect.equals(
               proxyHops,
               request.headers[HttpHeaders.VIA][0].split(",").length);
         } else {
           Expect.isNull(request.headers[HttpHeaders.VIA]);
         }
         var body = new StringBuffer();
         request.listen(
             (data) {
-              body.add(new String.fromCharCodes(data));
+              body.write(new String.fromCharCodes(data));
             },
             onDone: () {
               String path = request.uri.path.substring(1);
               String content = "$path$path$path";
               Expect.equals(content, body.toString());
               response.addString(request.uri.path);
               response.close();
             });
       });
       return x.future;
@@ skipping 303 matching lines @@
 main() {
   InitializeSSL();
   testInvalidProxy();
   testDirectProxy();
   testProxy();
   testProxyChain();
   // This test is not normally run. It can be used for locally testing
   // with a real proxy server (e.g. Apache).
   //testRealProxy();
 }