Include file path in JSON error when possible.

sdk/lib/_internal/pub/test/global/run/runs_script_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 '../../../lib/src/exit_codes.dart' as exit_codes;
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('runs a script in an activated package', () {
     servePackages([
       packageMap("foo", "1.0.0")
     ], contents: [
       d.dir("bin", [
         d.file("script.dart", "main(args) => print('ok');")
       ])
     ]);
 
     schedulePub(args: ["global", "activate", "foo"]);
 
     var pub = pubRun(global: true, args: ["foo", "script"]);
     pub.stdout.expect("ok");
     pub.shouldExit();
   });
 }