Print the stack traces for Future expectations that raise exceptions.

lib/unittest/core_matchers.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.
 
 
 /**
  * Returns a matcher that matches empty strings, maps or collections.
  */
 final Matcher isEmpty = const _Empty();
 
@@ skipping 262 matching lines @@
 
   bool matches(item) {
     if (item is Future) {
       // Queue up an asynchronous expectation that validates when the future
       // completes.
       item.onComplete(expectAsync1((future) {
         if (future.hasValue) {
           expect(false, reason:
               "Expected future to fail, but succeeded with '${future.value}'.");
         } else if (_matcher != null) {
-          expect(future.exception, _matcher);
+          var reason;
+          if (future.stackTrace != null) {
+            var stackTrace = future.stackTrace.toString();
+            stackTrace = "  ${stackTrace.replaceAll("\n", "\n  ")}";
+            reason = "Actual exception trace:\n$stackTrace";
+          }
+          expect(future.exception, _matcher, reason: reason);
         }
       }));
 
       // It hasn't failed yet.
       return true;
     }
 
     try {
       item();
       return false;
@@ skipping 287 matching lines @@
   final _matcher;
   final String _description;
 
   const _Predicate(this._matcher, this._description);
 
   bool matches(item) => _matcher(item);
 
   Description describe(Description description) =>
       description.add(_description);
 }