Respect top-level @TestOn declarations.

test/runner/browser/runner_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/util/exit_codes.dart' as exit_codes;
 import 'package:unittest/unittest.dart';
 
 import '../../io.dart';
 
 String _sandbox;
 
@@ skipping 137 matching lines @@
 }
 """);
       var result = _runUnittest(["-p", "chrome", "-p", "vm", "test.dart"]);
       expect(result.exitCode, equals(1));
     });
   });
 }
 
 ProcessResult _runUnittest(List<String> args) =>
     runUnittest(args, workingDirectory: _sandbox);