Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/language/string_interpolation_and_buffer.dart

Index: tests/language/string_interpolation_and_buffer.dart
diff --git a/tests/language/string_interpolation_and_buffer.dart b/tests/language/string_interpolation_and_buffer.dart
index 1ce359dc30b0f4bde01100f52cb78ce7cd6a4499..ddcdd4eeef6aa645504b04afcda847e2436ee8c7 100644
--- a/tests/language/string_interpolation_and_buffer.dart
+++ b/tests/language/string_interpolation_and_buffer.dart
@@ -41,13 +41,13 @@ main() {
     var sb;
     if (checkedMode && object != null) {
       try {
-        sb = new StringBuffer().add(wrap(object));
+        sb = new StringBuffer()..write(wrap(object));
       } on TypeError {
         return 'Error';
       }
     } else {
       try {
-        sb = new StringBuffer().add(wrap(object));
+        sb = new StringBuffer()..write(wrap(object));
       } on ArgumentError {
         return 'Error';
       }