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..b3a74d13fda72879a809c53e0a8689391f0f15d0 100644
--- a/pkg/scheduled_test/test/descriptor/nothing_test.dart
+++ b/pkg/scheduled_test/test/descriptor/nothing_test.dart
@@ -54,11 +54,10 @@ void main() {
     });
 
     test('test 2', () {
-      expect(errors, everyElement(new isInstanceOf<ScheduleError>()));
-      expect(errors.length, equals(1));
-      expect(errors.first.error,
+      expect(errors.single, new isInstanceOf<ScheduleError>());
+      expect(errors.single.error.toString(),
           matches(r"^Expected nothing to exist at '[^']+[\\/]name.txt', but "
-              r"found a file\.$"));
+                  r"found a file\.$"));
     });
   }, passing: ['test 2']);
 
@@ -76,11 +75,10 @@ void main() {
     });
 
     test('test 2', () {
-      expect(errors, everyElement(new isInstanceOf<ScheduleError>()));
-      expect(errors.length, equals(1));
-      expect(errors.first.error,
+      expect(errors.single, new isInstanceOf<ScheduleError>());
+      expect(errors.single.error.toString(),
           matches(r"^Expected nothing to exist at '[^']+[\\/]dir', but found a "
               r"directory\.$"));
     });
   }, passing: ['test 2']);
-}
+}