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..aabdbbaea7e830a5b8bc0894bba81b464e6da5f2 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 currentSchedule;
 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 Trace stackTrace;
 
   Task(fn(), String description, TaskQueue queue)
     : this._(fn, description, queue, null, queue.contents.length);
@@ -69,7 +70,9 @@ class Task {
   Task._child(fn(), String description, Task parent)
     : this._(fn, description, parent.queue, parent, parent.children.length);
 
-  Task._(fn(), this.description, this.queue, this.parent, this._id) {
+  Task._(fn(), this.description, TaskQueue queue, this.parent, this._id)
+      : queue = queue,
+        stackTrace = queue.captureStackTraces ? new Trace.current() : null {
     this.fn = () {
       if (state != TaskState.WAITING) {
         throw new StateError("Can't run $state task '$this'.");
@@ -121,8 +124,12 @@ class Task {
   String toString() => description == null ? "#$_id" : description;
 
   String toStringWithStackTrace() {
-    var stackString = prefixLines(terseTraceString(stackTrace));
-    return "$this\n\nStack trace:\n$stackString";
+    var result = toString();
+    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.