Remove deprecated StringBuffer.add, addAll and addCharCode.

utils/css/tool.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.
 
 library csstool;
 
 import 'dart:io';
 import 'css.dart';
 import '../lib/file_system.dart';
 import '../lib/file_system_vm.dart';
@@ skipping 54 matching lines @@
           new SourceFile(sourceFullFn, source), 0, files, sourcePath);
       stylesheet = parser.parse();
     });
 
     printStats(elapsed, sourceFullFn);
 
     StringBuffer buff = new StringBuffer(
       '/* File generated by SCSS from source ${sourceFilename}\n' +
       ' * Do not edit.\n' +
       ' */\n\n');
-    buff.add(stylesheet.toString());
+    buff.write(stylesheet.toString());
 
     files.writeString(outputFullFn, buff.toString());
     print("Generated file ${outputFullFn}");
 
     // Generate CSS.dart file.
     String genDartClassFile = Generate.dartClass(files, outPath, stylesheet,
         sourceFilename);
     print("Generated file ${genDartClassFile}");
   }
 }