Respect top-level @TestOn declarations.

test/runner/loader_test.dart

 // Copyright (c) 2015, 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.
 
+@TestOn("vm")
+
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:unittest/src/backend/state.dart';
 import 'package:unittest/src/backend/test_platform.dart';
 import 'package:unittest/src/runner/loader.dart';
 import 'package:unittest/unittest.dart';
 
 import '../io.dart';
 import '../utils.dart';
@@ skipping 62 matching lines @@
     });
 
     test("can load and run a failing test", () {
       var liveTest = suite.tests[1].load(suite);
       expectSingleFailure(liveTest);
       return liveTest.run().whenComplete(() => liveTest.close());
     });
 
     test("throws a nice error if the package root doesn't exist", () {
       var loader = new Loader([TestPlatform.vm]);
-      expect(() {
-        try {
-          loader.loadFile(p.join(_sandbox, 'a_test.dart'));
-        } finally {
-          loader.close();
-        }
-      }, throwsA(isLoadException(
-          "Directory ${p.join(_sandbox, 'packages')} does not exist.")));
+      expect(
+          loader.loadFile(p.join(_sandbox, 'a_test.dart'))
+              .whenComplete(loader.close),
+          throwsA(isLoadException(
+              "Directory ${p.join(_sandbox, 'packages')} does not exist.")));
     });
   });
 
   group(".loadDir()", () {
     test("ignores non-Dart files", () {
       new File(p.join(_sandbox, 'a_test.txt')).writeAsStringSync(_tests);
       expect(_loader.loadDir(_sandbox), completion(isEmpty));
     });
 
     test("ignores files in packages/ directories", () {
@@ skipping 33 matching lines @@
 
       test("can run tests in those suites", () {
         var suite = suites.firstWhere((suite) => suite.path.contains("a_test"));
         var liveTest = suite.tests[1].load(suite);
         expectSingleFailure(liveTest);
         return liveTest.run().whenComplete(() => liveTest.close());
       });
     });
   });
 }