Change package scheduled_test to throw TestFailures rather than strings.

pkg/scheduled_test/test/descriptor/nothing_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 import 'package:scheduled_test/descriptor.dart' as d;
 import 'package:scheduled_test/scheduled_test.dart';
 
@@ skipping 38 matching lines @@
         errors = currentSchedule.errors;
       });
 
       d.file('name.txt', 'contents').create();
       d.nothing('name.txt').validate();
     });
 
     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\.$"));
     });
   }, passing: ['test 2']);
 
   expectTestsPass("nothing().validate() fails if there's a directory", () {
     var errors;
     test('test 1', () {
       scheduleSandbox();
 
       currentSchedule.onException.schedule(() {
         errors = currentSchedule.errors;
       });
 
       d.dir('dir').create();
       d.nothing('dir').validate();
     });
 
     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']);
-}
+}