Remove deprecated StringBuffer.add, addAll and addCharCode.

utils/css/tool.dart

Index: utils/css/tool.dart
diff --git a/utils/css/tool.dart b/utils/css/tool.dart
index 6a96b50c211edf20d538c2ff98f4f757070a5280..cad31a27e5575a25d669a5a09827e0595893adbb 100644
--- a/utils/css/tool.dart
+++ b/utils/css/tool.dart
@@ -72,7 +72,7 @@ void main() {
       '/* 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}");