Remove deprecated StringBuffer.add, addAll and addCharCode.

utils/template/tool.dart

Index: utils/template/tool.dart
diff --git a/utils/template/tool.dart b/utils/template/tool.dart
index 97e1ac6e5617f3cc549cb6ed6e27fca846e9d602..59a8462f9826219b86d3598f5d0c5cb4ffa9ea9d 100644
--- a/utils/template/tool.dart
+++ b/utils/template/tool.dart
@@ -93,7 +93,7 @@ void main() {
     if (world.errors == 0) {
       // Generate the Dart class(es) for all template(s).
       codegenElapsed = time(() {
-        code.add(Codegen.generate(templates, outFilename));
+        code.write(Codegen.generate(templates, outFilename));
       });
     }