Implement custom status file expectations.

pkg/testing/lib/testing.dart

 // Copyright (c) 2016, 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.md file.
 
 library testing;
 
 export 'dart:async' show
     Future;
 
 export 'src/discover.dart';
 
 export 'src/test_description.dart';
 
 export 'src/chain.dart' show
     Chain,
     ChainContext,
     Result,
     Step;
 
 export 'src/stdio_process.dart' show
     StdioProcess;
 
 export 'src/run.dart' show
     run,
     runMe;
+
+export 'src/expectation.dart' show
+    Expectation,
+    ExpectationSet;