Update pub to use the VM's new bad-snapshot exit code.

sdk/lib/_internal/pub/test/global/activate/outdated_binstub_test.dart

 // Copyright (c) 2014, 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:io';
 import 'dart:async';
 
 import 'package:path/path.dart' as p;
 import 'package:scheduled_test/scheduled_process.dart';
 import 'package:scheduled_test/scheduled_stream.dart';
@@ skipping 32 matching lines @@
     d.dir(cachePath, [
       d.dir('bin', [
         d.file(binStubName('foo-script'), _OUTDATED_BINSTUB)
       ])
     ]).create();
 
     schedulePub(args: ["global", "activate", "foo"]);
 
     d.dir(cachePath, [
       d.dir('bin', [
-        // 255 is the VM's exit code upon seeing an out-of-date snapshot.
-        d.matcherFile(binStubName('foo-script'), contains("255"))
+        // 253 is the VM's exit code upon seeing an out-of-date snapshot.
+        d.matcherFile(binStubName('foo-script'), contains("253"))
       ])
     ]).validate();
   });
 }