Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/standalone/io/http_proxy_test.dart

Index: tests/standalone/io/http_proxy_test.dart
diff --git a/tests/standalone/io/http_proxy_test.dart b/tests/standalone/io/http_proxy_test.dart
index af8cdff4882b8fe658f1ecb1416feb2718a2f160..9a037a0db9235f871ef921cfbb333357b6cd966b 100644
--- a/tests/standalone/io/http_proxy_test.dart
+++ b/tests/standalone/io/http_proxy_test.dart
@@ -43,7 +43,7 @@ class Server {
         var body = new StringBuffer();
         request.listen(
             (data) {
-              body.add(new String.fromCharCodes(data));
+              body.write(new String.fromCharCodes(data));
             },
             onDone: () {
               String path = request.uri.path.substring(1);