Remove AsyncError with Expando.

pkg/scheduled_test/test/scheduled_test/capture_stack_traces_test.dart

Index: pkg/scheduled_test/test/scheduled_test/capture_stack_traces_test.dart
diff --git a/pkg/scheduled_test/test/scheduled_test/capture_stack_traces_test.dart b/pkg/scheduled_test/test/scheduled_test/capture_stack_traces_test.dart
index 6ea9e59f9baad2ee0864a3c807437a513ee8139a..25bba2043ddeef010546fd1955f435a39a4d0016 100644
--- a/pkg/scheduled_test/test/scheduled_test/capture_stack_traces_test.dart
+++ b/pkg/scheduled_test/test/scheduled_test/capture_stack_traces_test.dart
@@ -32,6 +32,7 @@ void main() {
   }, passing: ['test 2']);
 
   expectTestsPass('does not capture a stack trace if set to false', () {
+    var errorThrown = new Object();
     var error;
     test('test 1', () {
       currentSchedule.captureStackTraces = false;
@@ -39,7 +40,7 @@ void main() {
         error = currentSchedule.errors.first;
       });
 
-      schedule(() => throw 'error');
+      schedule(() => throw errorThrown);
     });
 
     test('test 2', () {