Move the status file parser into its own package.

tools/testing/dart/command.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 // We need to use the 'io' prefix here, otherwise io.exitCode will shadow
 // CommandOutput.exitCode in subclasses of CommandOutput.
 import 'dart:io' as io;
 
+import 'package:status_file/expectation.dart';
+
 import 'command_output.dart';
 import 'configuration.dart';
-import 'expectation.dart';
 import 'path.dart';
 import 'utils.dart';
 
 /// A command executed as a step in a test case.
 class Command {
   static Command contentShell(
       String executable,
       String htmlFile,
       List<String> options,
       List<String> dartFlags,
@@ skipping 533 matching lines @@
 
   void _buildHashCode(HashCodeBuilder builder) {
     super._buildHashCode(builder);
     builder.addJson(_link);
     builder.addJson(_target);
   }
 
   bool _equal(MakeSymlinkCommand other) =>
       super._equal(other) && _link == other._link && _target == other._target;
 }