Remove deprecated StringBuffer.add, addAll and addCharCode.

utils/css/tokenkind.dart

Index: utils/css/tokenkind.dart
diff --git a/utils/css/tokenkind.dart b/utils/css/tokenkind.dart
index 6112e82a3de9272531936abb6949e6afa99d06a2..fbcebdf095731952ff8a2f6d6d8bfe1af46b2f90 100644
--- a/utils/css/tokenkind.dart
+++ b/utils/css/tokenkind.dart
@@ -487,10 +487,10 @@ class TokenKind {
     StringBuffer invertResult = new StringBuffer();
     int paddings = minDigits - result.length;
     while (paddings-- > 0) {
-      invertResult.add('0');
+      invertResult.write('0');
     }
     for (int idx = result.length - 1; idx >= 0; idx--) {
-      invertResult.add(result[idx]);
+      invertResult.write(result[idx]);
     }
 
     return invertResult.toString();