Re-add exit_codes import to runner_test.

test/runner/runner_test.dart

 // Copyright (c) 2015, 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.
 
 @TestOn("vm")
 
 import 'dart:io';
 import 'dart:math' as math;
 
 import 'package:path/path.dart' as p;
+import 'package:test/src/util/exit_codes.dart' as exit_codes;
 import 'package:test/src/util/io.dart';
 import 'package:test/test.dart';
 
 import '../io.dart';
 
 String _sandbox;
 
 final _success = """
 import 'dart:async';
 
@@ skipping 417 matching lines @@
       });
     });
   });
 }
 
 ProcessResult _runUnittest(List<String> args) =>
     runUnittest(args, workingDirectory: _sandbox);
 
 ProcessResult _runDart(List<String> args) =>
     runDart(args, workingDirectory: _sandbox);