Redo StreamTransformers so they work with Stack traces.

tests/standalone/io/web_socket_pipe_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 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());
       });
     });
     WebSocket.connect("ws://127.0.0.1:${server.port}/").then((client) {
       var count = 0;
       next() {
         if (count < messages) {
           client.add("Hello");
@@ skipping 19 matching lines @@
 }
 
 void main() {
   testPipe(messages: 0, transform: false);
   testPipe(messages: 0, transform: true);
   testPipe(messages: 1, transform: false);
   testPipe(messages: 1, transform: true);
   testPipe(messages: 10, transform: false);
   testPipe(messages: 10, transform: true);
 }