Respect top-level @TestOn declarations.

test/runner/browser/compiler_pool_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 35 matching lines @@
     new File(testPath).writeAsStringSync("String main() => 12;\n");
 
     var result = _runUnittest(["--color", "-p", "chrome", "test.dart"]);
     expect(result.stdout, contains('\u001b[35m'));
     expect(result.exitCode, equals(exit_codes.data));
   });
 }
 
 ProcessResult _runUnittest(List<String> args) =>
     runUnittest(args, workingDirectory: _sandbox);