Remove deprecated StringBuffer.add, addAll and addCharCode.

utils/peg/pegparser.dart

Index: utils/peg/pegparser.dart
diff --git a/utils/peg/pegparser.dart b/utils/peg/pegparser.dart
index bd329c71f8120dd8beb16e3818aa2d5ed4181dd5..ba1526b958a2eb9eb5c9b037e0e290279426f310 100644
--- a/utils/peg/pegparser.dart
+++ b/utils/peg/pegparser.dart
@@ -600,14 +600,14 @@ _Rule _compileMultiRule(List rules,
 
 String _formatMultiRule(String functor, List rules) {
   var sb = new StringBuffer(functor);
-  sb.add('(');
+  sb.write('(');
   var separator = '';
   for (var rule in rules) {
-    sb.add(separator);
-    sb.add(rule);
+    sb.write(separator);
+    sb.write(rule);
     separator = ',';
   }
-  sb.add(')');
+  sb.write(')');
   return sb.toString();
 }