Support an @OnPlatform annotation.

lib/test.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 test;
 
 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/backend/test_platform.dart';
 import 'src/frontend/timeout.dart';
 import 'src/runner/reporter/no_io_compact.dart';
 import 'src/utils.dart';
 
 export 'package:matcher/matcher.dart';
 
 export 'src/frontend/expect.dart';
 export 'src/frontend/expect_async.dart';
 export 'src/frontend/future_matchers.dart';
+export 'src/frontend/on_platform.dart';
 export 'src/frontend/prints_matcher.dart';
 export 'src/frontend/skip.dart';
 export 'src/frontend/test_on.dart';
 export 'src/frontend/throws_matcher.dart';
 export 'src/frontend/throws_matchers.dart';
 export 'src/frontend/timeout.dart';
 
 /// The global declarer.
 ///
 /// This is used if a test file is run directly, rather than through the runner.
@@ skipping 134 matching lines @@
 
 /// Handle an error that occurs outside of any test.
 void handleExternalError(error, String message, [stackTrace]) {
   // TODO(nweiz): handle this better.
   registerException(error, stackTrace);
 }
 
 /// Registers an exception that was caught for the current test.
 void registerException(error, [StackTrace stackTrace]) =>
     Invoker.current.handleError(error, stackTrace);