Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/standalone/io/file_input_stream_test.dart

Index: tests/standalone/io/file_input_stream_test.dart
diff --git a/tests/standalone/io/file_input_stream_test.dart b/tests/standalone/io/file_input_stream_test.dart
index e9de0c5eea182a79522b711a5d312e49649ca355..babd9489bd874ac1a27aa774f1cefeb2320d8523 100644
--- a/tests/standalone/io/file_input_stream_test.dart
+++ b/tests/standalone/io/file_input_stream_test.dart
@@ -53,7 +53,7 @@ int writeLongFileSync(File file) {
   file.createSync();
   StringBuffer buffer = new StringBuffer();
   for (var i = 0; i < 10000; i++) {
-    buffer.add("Hello, world");
+    buffer.write("Hello, world");
   }
   file.writeAsStringSync(buffer.toString());
   var length = file.lengthSync();