TestFailure + Error baseclass = stack traces!

pkg/unittest/lib/src/expect.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.
 
 part of matcher;
 
 /** The objects thrown by the default failure handler. */
-class TestFailure {
-  String _message;
+class TestFailure extends Error{
+  final String message;
 
-  get message => _message;
-  set message(String value) => _message = value;
+  TestFailure(this.message);
 
-  TestFailure(String message) : _message = message;
-
-  String toString() => _message;
+  String toString() => message;
 }
 
 /**
  * Useful utility for nesting match states.
  */
 
 void addStateInfo(Map matchState, Map values) {
   var innerState = new Map.from(matchState);
   matchState.clear();
   matchState['state'] = innerState;
   matchState.addAll(values);
 }
 
 /**
  * Some matchers, like those for Futures and exception testing,
  * can fail in asynchronous sections, and throw exceptions.
- * A user of this library will typically want to catch and handle 
+ * A user of this library will typically want to catch and handle
  * such exceptions. The [wrapAsync] property is a function that
  * can wrap callbacks used by these Matchers so that they can be
  * used safely. For example, the unittest library will set this
  * to be expectAsync1. By default this is an identity function.
  */
 Function wrapAsync = (f, [id]) => f;
 
 /**
  * This is the main assertion function. It asserts that [actual]
  * matches the [matcher]. [reason] is optional and is typically not
@@ skipping 126 matching lines @@
  * formatter is returned; this allows custom expect handlers to easily
  * get a reference to the default formatter.
  */
 ErrorFormatter configureExpectFormatter([ErrorFormatter formatter = null]) {
   if (formatter == null) {
     formatter = _defaultErrorFormatter;
   }
   return _assertErrorFormatter = formatter;
 }