Respect top-level @TestOn declarations.

lib/unittest.dart

 // Copyright (c) 2013, 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;
 
 import 'dart:async';
 
 import 'package:path/path.dart' as p;
 
 import 'src/backend/declarer.dart';
 import 'src/backend/invoker.dart';
 import 'src/backend/suite.dart';
 import 'src/deprecated/configuration.dart';
 import 'src/deprecated/test_case.dart';
 import 'src/runner/reporter/no_io_compact.dart';
 
 export 'package:matcher/matcher.dart';
 
 export 'src/deprecated/configuration.dart';
 export 'src/deprecated/simple_configuration.dart';
 export 'src/deprecated/test_case.dart';
 export 'src/frontend/expect.dart';
 export 'src/frontend/expect_async.dart';
 export 'src/frontend/future_matchers.dart';
 export 'src/frontend/prints_matcher.dart';
+export 'src/frontend/test_on.dart';
 export 'src/frontend/throws_matcher.dart';
 export 'src/frontend/throws_matchers.dart';
 
 /// The global declarer.
 ///
 /// This is used if a test file is run directly, rather than through the runner.
 Declarer _globalDeclarer;
 
 /// Gets the declarer for the current scope.
 ///
@@ skipping 106 matching lines @@
 void setSoloTest(int id) {}
 
 @deprecated
 void enableTest(int id) {}
 
 @deprecated
 void disableTest(int id) {}
 
 @deprecated
 withTestEnvironment(callback()) => callback();