Remove deprecated StringBuffer.add, addAll and addCharCode.

utils/lib/file_system_memory.dart

 // Copyright (c) 2011, 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.
 
 // TODO(terry): Investigate common library for file I/O shared between frog and tools.
 
 library file_system_memory;
 
 import 'file_system.dart';
 
 /**
  * [FileSystem] implementation a memory buffer.
  */
 class MemoryFileSystem implements FileSystem {
   StringBuffer buffer;
 
   MemoryFileSystem() : this.buffer = new StringBuffer();
 
   void writeString(String outfile, String text) {
-    buffer.add(text);
+    buffer.write(text);
   }
 
   String readAll(String filename) {
     return buffer.toString();
   }
 
   bool fileExists(String filename) {
     return true;
   }
 
   void createDirectory(String path, [bool recursive]) {
     // TODO(terry): To be implement.
     throw 'createDirectory() is not implemented by MemoryFileSystem yet.';
   }
 
   void removeDirectory(String path, [bool recursive]) {
     // TODO(terry): To be implement.
     throw 'removeDirectory() is not implemented by MemoryFileSystem yet.';
   }
 }