Redo StreamTransformers so they work with Stack traces.

tests/standalone/io/web_socket_pipe_test.dart

Index: tests/standalone/io/web_socket_pipe_test.dart
diff --git a/tests/standalone/io/web_socket_pipe_test.dart b/tests/standalone/io/web_socket_pipe_test.dart
index e182a40389dfed0a2cd83c41b316fcac853dfa42..b487b7473bd6bc5aa7a4ccd69d65ac9d82e8ecf1 100644
--- a/tests/standalone/io/web_socket_pipe_test.dart
+++ b/tests/standalone/io/web_socket_pipe_test.dart
@@ -11,23 +11,20 @@ import "package:expect/expect.dart";
 import "dart:async";
 import "dart:io";
 
-class IdentityTransformer extends StreamEventTransformer {
-  void handleData(data, sink) => sink.add(data);
-}
-
-class ReverseStringTransformer extends StreamEventTransformer {
-  void handleData(String data, sink) {
-    var sb = new StringBuffer();
-    for (int i = data.length - 1; i >= 0; i--) sb.write(data[i]);
-    sink.add(sb.toString());
-  }
+createReverseStringTransformer() {
+  return new StreamTransformer.fromHandlers(
+      handleData: (String data, sink) {
+        var sb = new StringBuffer();
+        for (int i = data.length - 1; i >= 0; i--) sb.write(data[i]);
+        sink.add(sb.toString());
+      });
 }
 
 testPipe({int messages, bool transform}) {
   HttpServer.bind("127.0.0.1", 0).then((server) {
     server.listen((request) {
       WebSocketTransformer.upgrade(request).then((websocket) {
-        (transform ? websocket.transform(new ReverseStringTransformer())
+        (transform ? websocket.transform(createReverseStringTransformer())
                    : websocket)
             .pipe(websocket)
             .then((_) => server.close());