Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/language/final_field_initialization_order_test.dart

Index: tests/language/final_field_initialization_order_test.dart
diff --git a/tests/language/final_field_initialization_order_test.dart b/tests/language/final_field_initialization_order_test.dart
index bd7c05369920f060142f14eef79a2b73d46dcda0..87350463d5198d60dab60a8170d3c64a50cb1626 100644
--- a/tests/language/final_field_initialization_order_test.dart
+++ b/tests/language/final_field_initialization_order_test.dart
@@ -10,7 +10,7 @@ int counter = 0;
 class Mark {
   static StringBuffer buffer;
   Mark(value) {
-    buffer.add('$value.');
+    buffer.write('$value.');
   }
 }