Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/isolate/mandel_isolate_test.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.
 
 library MandelIsolateTest;
 import 'dart:async';
 import 'dart:isolate';
 import 'dart:math';
 import '../../pkg/unittest/lib/unittest.dart';
 
@@ skipping 49 matching lines @@
   void _validateResult() {
     // TODO(ngeoffray): Implement this.
     _validated.complete(true);
   }
 
   void _printResult() {
     var output = new StringBuffer();
     for (int i = 0; i < _result.length; i++) {
       List<int> line = _result[i];
       for (int j = 0; j < line.length; j++) {
-        if (line[j] < 10) output.add("0");
-        output.add(line[j]);
+        if (line[j] < 10) output.write("0");
+        output.write(line[j]);
       }
-      output.add("\n");
+      output.write("\n");
     }
     // print(output);
   }
 
   List<List<int>> _result;
   List<LineProcessorClient> _lineProcessedBy;
   int _sent;
   int _missing;
   Completer<bool> _validated;
 }
@@ skipping 50 matching lines @@
 void processLines() {
   port.receive((message, SendPort replyTo) {
     if (message == TERMINATION_MESSAGE) {
       assert(replyTo == null);
       port.close();
     } else {
       replyTo.send(processLine(message), null);
     }
   });
 }