Display which nested tasks are running when an error occurs in a 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:unittest/unittest.dart' as unittest;
 
 import 'mock_clock.dart' as mock_clock;
 import 'schedule_error.dart';
 import 'substitute_future.dart';
 import 'task.dart';
+import 'value_future.dart';
 
 /// The schedule of tasks to run for a single test. This has three separate task
 /// queues: [tasks], [onComplete], and [onException]. It also provides
 /// visibility into the current state of the schedule.
 class Schedule {
   /// The main task queue for the schedule. These tasks are run before the other
   /// queues and generally constitute the main test body.
   TaskQueue get tasks => _tasks;
   TaskQueue _tasks;
 
@@ skipping 499 matching lines @@
   ///
   /// [highlight] must be a task in this queue.
   String generateTree([Task highlight]) {
     assert(highlight == null || highlight.queue == this);
     return _contents.map((task) {
       var lines = task.toString().split("\n");
       var firstLine = task == highlight ?
           "> ${lines.first}" : "* ${lines.first}";
       lines = new List.from(lines.skip(1).map((line) => "| $line"));
       lines.insertRange(0, 1, firstLine);
+
+      if (task == highlight && !task.children.isEmpty) {
+        for (var child in task.children) {
+          var prefix = ">";
+          if (child.state == TaskState.ERROR) {
+            prefix = "X";
+          } else if (child.state == TaskState.SUCCESS) {
+            prefix = "*";
+          }
+
+          var childLines = child.toString().split("\n");
+          lines.add("  $prefix ${childLines.first}");
+          lines.addAll(childLines.skip(1).map((line) => "  | $line"));
+        }
+      }
+
       return lines.join("\n");
     }).join("\n");
   }
 }