Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/language/field_initialization_order_test.dart

Index: tests/language/field_initialization_order_test.dart
diff --git a/tests/language/field_initialization_order_test.dart b/tests/language/field_initialization_order_test.dart
index 68908957ff627ae3c5bd0e270a0a67fc5867223f..3bf24b6038ec1d32bf337e120a80b1ce3d370cd8 100644
--- a/tests/language/field_initialization_order_test.dart
+++ b/tests/language/field_initialization_order_test.dart
@@ -9,7 +9,7 @@ int counter = 0;
 class Mark {
   static StringBuffer buffer;
   Mark(value) {
-    buffer.add('$value.');
+    buffer.write('$value.');
   }
 }