Redo StreamTransformers so they work with Stack traces.

tests/lib/async/event_helper.dart

 // Copyright (c) 2012, 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.
 
 library event_helper;
 
 import 'dart:async';
 
 abstract class Event {
   void replay(EventSink sink);
@@ skipping 62 matching lines @@
   /** Capture events from a stream into a new [Events] object. */
   factory Events.capture(Stream stream,
                          { bool cancelOnError: false }) = CaptureEvents;
 
   // EventSink interface.
   void add(var value) {
     if (trace) print("Events#$hashCode: add($value)");
     events.add(new DataEvent(value));
   }
 
-  void addError(error) {
+  void addError(error, [StackTrace stackTrace]) {
     if (trace) print("Events#$hashCode: addError($error)");
     events.add(new ErrorEvent(error));
   }
 
   void close() {
     if (trace) print("Events#$hashCode: close()");
     events.add(const DoneEvent());
   }
 
   // Error helper for creating errors manually..
-  void error(var value) { addError(value); }
+  void error(var value, [StackTrace stackTrace]) {
+    addError(value, stackTrace);
+  }
 
   /** Replay the captured events on a sink. */
   void replay(EventSink sink) {
     for (int i = 0; i < events.length; i++) {
       events[i].replay(sink);
     }
   }
 
   /**
    * Create a new [Events] with the same captured events.
@@ skipping 64 matching lines @@
   void resume() {
     if (trace) print("Events#$hashCode: resume");
     subscription.resume();
   }
 
   void onDone(void action()) {
     if (trace) print("Events#$hashCode: onDone");
     onDoneSignal.future.whenComplete(action);
   }
 }