Add flag to disable capturing stack traces.

pkg/scheduled_test/lib/src/schedule_error.dart

Index: pkg/scheduled_test/lib/src/schedule_error.dart
diff --git a/pkg/scheduled_test/lib/src/schedule_error.dart b/pkg/scheduled_test/lib/src/schedule_error.dart
index 3df208a01d353dc8821feb4976f70087f6c7bddc..48ed00210c7bec26a6f9a9aefd5456b933f56588 100644
--- a/pkg/scheduled_test/lib/src/schedule_error.dart
+++ b/pkg/scheduled_test/lib/src/schedule_error.dart
@@ -8,6 +8,7 @@ import 'dart:async';
 
 import 'package:stack_trace/stack_trace.dart';
 
+import '../scheduled_test.dart' show captureStackTraces;
 import 'schedule.dart';
 import 'task.dart';
 import 'utils.dart';
@@ -50,7 +51,7 @@ class ScheduleError extends AsyncError {
       error = error.error;
     }
 
-    if (stackTrace == null) {
+    if (captureStackTraces && stackTrace == null) {
       try {
         throw '';
       } catch (_, thrownStackTrace) {
@@ -88,9 +89,11 @@ class ScheduleError extends AsyncError {
       result.write('ScheduleError: "$errorString"\n');
     }
 
-    result.write('Stack trace:\n');
-    result.write(prefixLines(terseTraceString(stackTrace)));
-    result.write("\n\n");
+    if (stackTrace != null) {
+      result.write('Stack trace:\n');
+      result.write(prefixLines(terseTraceString(stackTrace)));
+      result.write("\n\n");
+    }
 
     if (task != null) {
       result.write('Error detected during task in queue "$queue":\n');