Use the stack_trace library in scheduled_test.

pkg/scheduled_test/lib/src/schedule.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 schedule;
 
 import 'dart:async';
 import 'dart:collection';
 
+import 'package:stack_trace/stack_trace.dart';
 import 'package:unittest/unittest.dart' as unittest;
 
 import 'mock_clock.dart' as mock_clock;
 import 'schedule_error.dart';
 import 'substitute_future.dart';
 import 'task.dart';
 import 'utils.dart';
 import 'value_future.dart';
 
 /// The schedule of tasks to run for a single test. This has three separate task
@@ skipping 468 matching lines @@
   Function _wrapAsync(fn(arg), String description) {
     assert(_schedule.state == ScheduleState.SET_UP || isRunning);
 
     // It's possible that the queue timed out before [fn] finished.
     bool _timedOut() =>
       _schedule.currentQueue != this || pendingCallbacks.isEmpty;
 
     if (description == null) {
       description = "Out-of-band operation #${_totalCallbacks}";
     }
+    var stackString = prefixLines(terseTraceString(new Trace.current()));
+    description = "$description\n\nStack trace:\n$stackString";
     _totalCallbacks++;
 
     _pendingCallbacks.add(description);
     return (arg) {
       try {
         return fn(arg);
       } catch (e, stackTrace) {
         var error = new ScheduleError.from(
             _schedule, e, stackTrace: stackTrace);
         if (_timedOut()) {
@@ skipping 43 matching lines @@
 
   String toString() => name;
 
   /// Returns a detailed representation of the queue as a tree of tasks. If
   /// [highlight] is passed, that task is specially highlighted.
   ///
   /// [highlight] must be a task in this queue.
   String generateTree([Task highlight]) {
     assert(highlight == null || highlight.queue == this);
     return _contents.map((task) {
-      var taskString = prefixLines(task.toString(),
+      var taskString = task == highlight
+          ? task.toStringWithStackTrace()
+          : task.toString();
+      taskString = prefixLines(taskString,
           firstPrefix: task == highlight ? "> " : "* ");
 
       if (task == highlight && !task.children.isEmpty) {
         var childrenString = task.children.map((child) {
           var prefix = ">";
           if (child.state == TaskState.ERROR) {
             prefix = "X";
           } else if (child.state == TaskState.SUCCESS) {
             prefix = "*";
           }
 
-          return prefixLines(child.toString(),
+          var childString = prefix == "*"
+              ? child.toString()
+              : child.toStringWithStackTrace();
+          return prefixLines(childString,
               firstPrefix: "  $prefix ", prefix: "  | ");
         }).join('\n');
         taskString = '$taskString\n$childrenString';
       }
 
       return taskString;
     }).join("\n");
   }
 }