Change package scheduled_test to throw TestFailures rather than strings.

pkg/scheduled_test/test/descriptor/nothing_test.dart

Index: pkg/scheduled_test/test/descriptor/nothing_test.dart
diff --git a/pkg/scheduled_test/test/descriptor/nothing_test.dart b/pkg/scheduled_test/test/descriptor/nothing_test.dart
index 5d145923ffd987e9616aa43f4fb7b935bb62210d..59e3f051f385ce3926a0dcb9fe8f7e02625e6cda 100644
--- a/pkg/scheduled_test/test/descriptor/nothing_test.dart
+++ b/pkg/scheduled_test/test/descriptor/nothing_test.dart
@@ -56,7 +56,8 @@ void main() {
     test('test 2', () {
       expect(errors, everyElement(new isInstanceOf<ScheduleError>()));
       expect(errors.length, equals(1));
-      expect(errors.first.error,
+      expect(errors.first.error, new isInstanceOf<TestFailure>());
+      expect(errors.first.error.message,
           matches(r"^Expected nothing to exist at '[^']+[\\/]name.txt', but "
               r"found a file\.$"));
     });
@@ -78,9 +79,10 @@ void main() {
     test('test 2', () {
       expect(errors, everyElement(new isInstanceOf<ScheduleError>()));
       expect(errors.length, equals(1));
-      expect(errors.first.error,
+      expect(errors.first.error, new isInstanceOf<TestFailure>());
+      expect(errors.first.error.message,
           matches(r"^Expected nothing to exist at '[^']+[\\/]dir', but found a "
               r"directory\.$"));
     });
   }, passing: ['test 2']);
-}
+}