Respect top-level @TestOn declarations.

lib/src/util/io.dart

Index: lib/src/util/io.dart
diff --git a/lib/src/util/io.dart b/lib/src/util/io.dart
index 9a00ad642be035a69a0a350f4eebd6ecb4b48bc3..c14e32f56bc86eda13003b87f9b34482c18a9654 100644
--- a/lib/src/util/io.dart
+++ b/lib/src/util/io.dart
@@ -10,12 +10,22 @@ 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);