Respect top-level @TestOn declarations.

test/runner/isolate_listener_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:async';
 import 'dart:isolate';
 
 import 'package:unittest/src/backend/invoker.dart';
 import 'package:unittest/src/backend/live_test.dart';
 import 'package:unittest/src/backend/state.dart';
 import 'package:unittest/src/backend/suite.dart';
 import 'package:unittest/src/runner/vm/isolate_listener.dart';
 import 'package:unittest/src/runner/vm/isolate_test.dart';
 import 'package:unittest/src/util/io.dart';
@@ skipping 354 matching lines @@
   IsolateListener.start(sendPort, () => () {
     test("multiple errors", () {
       Invoker.current.addOutstandingCallback();
       new Future(() => throw "one");
       new Future(() => throw "two");
       new Future(() => throw "three");
       new Future(() => throw "four");
     });
   });
 }