Respect top-level @TestOn declarations.

test/backend/platform_selector/evaluate_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:unittest/unittest.dart';
 import 'package:unittest/src/backend/operating_system.dart';
 import 'package:unittest/src/backend/platform_selector.dart';
 import 'package:unittest/src/backend/test_platform.dart';
 
 void main() {
   test("new PlatformSelector.parse() disallows invalid variables", () {
     expect(() => new PlatformSelector.parse("undefined"),
@@ skipping 116 matching lines @@
 /// Returns the result of evaluating [expression] on [platform] and [os].
 ///
 /// [platform] defaults to [TestPlatform.vm]; [os] defaults to the current
 /// operating system.
 bool _eval(String expression, {TestPlatform platform, OperatingSystem os}) {
   if (platform == null) platform = TestPlatform.vm;
   if (os == null) os = OperatingSystem.findByIoName(Platform.operatingSystem);
   var selector = new PlatformSelector.parse(expression);
   return selector.evaluate(platform, os: os);
 }