Add flag to disable capturing stack traces.

pkg/scheduled_test/lib/src/task.dart

Index: pkg/scheduled_test/lib/src/task.dart
diff --git a/pkg/scheduled_test/lib/src/task.dart b/pkg/scheduled_test/lib/src/task.dart
index 4ccb201f5fbe85af5f5f3ac19a7fe26021075cea..63af96b1f750f449b164a28e1cbfb85bfb2a5398 100644
--- a/pkg/scheduled_test/lib/src/task.dart
+++ b/pkg/scheduled_test/lib/src/task.dart
@@ -9,6 +9,7 @@ import 'dart:collection';
 
 import 'package:stack_trace/stack_trace.dart';
 
+import '../scheduled_test.dart' show captureStackTraces;
 import 'future_group.dart';
 import 'schedule.dart';
 import 'utils.dart';
@@ -61,7 +62,7 @@ class Task {
   Future get result => _resultCompleter.future;
   final _resultCompleter = new Completer();
 
-  final stackTrace = new Trace.current();
+  final stackTrace = captureStackTraces ? new Trace.current() : null;
 
   Task(fn(), String description, TaskQueue queue)
     : this._(fn, description, queue, null, queue.contents.length);
@@ -121,8 +122,12 @@ class Task {
   String toString() => description == null ? "#$_id" : description;
 
   String toStringWithStackTrace() {
-    var stackString = prefixLines(terseTraceString(stackTrace));
-    return "$this\n\nStack trace:\n$stackString";
+    var result = "$this";

[nweiz, 2013/04/09 21:25:58]

I think I prefer `this.toString()` here.

[Bob Nystrom, 2013/04/10 21:52:00]

Done.

+    if (stackTrace != null) {
+      var stackString = prefixLines(terseTraceString(stackTrace));
+      result += "\n\nStack trace:\n$stackString";
+    }
+    return result;
   }
 
   /// Returns a detailed representation of [queue] with this task highlighted.