Respect top-level @TestOn declarations.

test/runner/browser/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 89 matching lines @@
 
       for (var suite in suites) {
         expect(suite.tests, hasLength(3));
         expect(suite.tests[0].name, equals("success"));
         expect(suite.tests[1].name, equals("failure"));
         expect(suite.tests[2].name, equals("error"));
       }
     }).whenComplete(loader.close);
   });
 }