Add flag to disable capturing stack traces.

pkg/scheduled_test/lib/src/schedule.dart

Index: pkg/scheduled_test/lib/src/schedule.dart
diff --git a/pkg/scheduled_test/lib/src/schedule.dart b/pkg/scheduled_test/lib/src/schedule.dart
index 88c26eb1539421ce86b6caa09a822d3d6d4b78bb..451c994569ed5aad70dd5db106aa5b7dcb5cbb59 100644
--- a/pkg/scheduled_test/lib/src/schedule.dart
+++ b/pkg/scheduled_test/lib/src/schedule.dart
@@ -12,6 +12,7 @@ import 'package:unittest/unittest.dart' as unittest;
 
 import 'mock_clock.dart' as mock_clock;
 import 'schedule_error.dart';
+import '../scheduled_test.dart' show captureStackTraces;
 import 'substitute_future.dart';
 import 'task.dart';
 import 'utils.dart';
@@ -309,7 +310,7 @@ class Schedule {
   /// are no duplicate errors, and that all errors are wrapped in
   /// [ScheduleError].
   void _addError(error) {
-    if (errors.contains(error)) return;
+    if (error is ScheduleError && errors.contains(error)) return;
     errors.add(new ScheduleError.from(this, error));
   }
 }
@@ -496,8 +497,12 @@ class TaskQueue {
     if (description == null) {
       description = "Out-of-band operation #${_totalCallbacks}";
     }
-    var stackString = prefixLines(terseTraceString(new Trace.current()));
-    description = "$description\n\nStack trace:\n$stackString";
+
+    if (captureStackTraces) {
+      var stackString = prefixLines(terseTraceString(new Trace.current()));
+      description += "\n\nStack trace:\n$stackString";
+    }
+
     _totalCallbacks++;
 
     _pendingCallbacks.add(description);