Use the stack_trace library in scheduled_test.

pkg/scheduled_test/lib/src/scheduled_future_matchers.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.
 
 library scheduled_future_matchers;
 
 import 'dart:async';
 
+import 'package:stack_trace/stack_trace.dart';
+
 import '../scheduled_test.dart';
 
 /// Matches a [Future] that completes successfully with a value. Note that this
 /// creates an asynchronous expectation. The call to `expect()` that includes
 /// this will return immediately and execution will continue. Later, when the
 /// future completes, the actual expectation will run.
 ///
 /// To test that a Future completes with an exception, you can use [throws] and
 /// [throwsA].
 ///
@@ skipping 34 matching lines @@
     if (description == null) {
       if (_matcher == null) {
         description = 'expect(..., completes)';
       } else {
         var matcherDescription = new StringDescription();
         _matcher.describe(matcherDescription);
         description = 'expect(..., completion($matcherDescription))';
       }
     }
 
+    var outerTrace = new Trace.current();
     currentSchedule.wrapFuture(item.then((value) {
-      if (_matcher != null) expect(value, _matcher);
+      if (_matcher == null) return;
+      try {

[Bob Nystrom, 2013/04/01 17:30:49]

Nit, but how about a blank line before the try?

[nweiz, 2013/04/01 20:46:03]

Done.

+        expect(value, _matcher);
+      } catch (e, stackTrace) {
+        throw new AsyncError(e, outerTrace);
+      }
     }), description);
 
     return true;
   }
 
   Description describe(Description description) {
     if (_matcher == null) {
       description.add('completes successfully');
     } else {
       description.add('completes to a value that ').addDescriptionOf(_matcher);
     }
     return description;
   }
 }