Respect top-level @TestOn declarations.

lib/src/util/io.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.
 
 library unittest.util.io;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:mirrors';
 
 import 'package:path/path.dart' as p;
 
+import '../backend/operating_system.dart';
 import '../runner/load_exception.dart';
 
 /// The root directory of the Dart SDK.
 final String sdkDir =
     p.dirname(p.dirname(Platform.executable));
 
+/// Returns the current operating system.
+final OperatingSystem currentOS = (() {
+  var name = Platform.operatingSystem;
+  var os = OperatingSystem.findByIoName(name);
+  if (os != null) return os;
+
+  throw new UnsupportedError('Unsupported operating system "$name".');
+})();
+
 /// The path to the `lib` directory of the `unittest` package.
 String libDir({String packageRoot}) {
   var pathToIo = libraryPath(#unittest.util.io, packageRoot: packageRoot);
   return p.dirname(p.dirname(p.dirname(pathToIo)));
 }
 
 /// Returns whether the current Dart version supports [Isolate.kill].
 final bool supportsIsolateKill = _supportsIsolateKill;
 bool get _supportsIsolateKill {
   // This isn't 100% accurate, since early 1.9 dev releases didn't support
@@ skipping 68 matching lines @@
 /// returned.
 String libraryPath(Symbol libraryName, {String packageRoot}) {
   var lib = currentMirrorSystem().findLibrary(libraryName);
   if (lib.uri.scheme != 'package') return p.fromUri(lib.uri);
 
   // TODO(nweiz): is there a way to avoid assuming this is being run next to a
   // packages directory?.
   if (packageRoot == null) packageRoot = p.absolute('packages');
   return p.join(packageRoot, p.fromUri(lib.uri.path));
 }