Change package scheduled_test to throw TestFailures rather than strings.

pkg/scheduled_test/lib/src/descriptor/nothing_descriptor.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.
 
 library descriptor.nothing;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 
 import '../../descriptor.dart';
 import '../../scheduled_test.dart';
 
 /// A descriptor that validates that no file exists with the given name.
 /// Creating this descriptor is a no-op and loading from it is invalid.
 class NothingDescriptor extends Descriptor {
   NothingDescriptor(String name)
       : super(name);
 
   Future create([String parent]) => new Future.value();
 
   Future validate([String parent]) => schedule(() => validateNow(parent),
       "validating '$name' doesn't exist");
 
   Future validateNow([String parent]) => new Future.sync(() {
     if (parent == null) parent = defaultRoot;
     var fullPath = path.join(parent, name);
     if (new File(fullPath).existsSync()) {
-      throw "Expected nothing to exist at '$fullPath', but found a file.";
+      fail("Expected nothing to exist at '$fullPath', but found a file.");
     } else if (new Directory(fullPath).existsSync()) {
-      throw "Expected nothing to exist at '$fullPath', but found a "
-          "directory.";
+      fail("Expected nothing to exist at '$fullPath', but found a directory.");
     } else {
       return;
     }
   });
 
   String describe() => "nothing at '$name'";
 }