Include file path in JSON error when possible.

sdk/lib/_internal/pub/test/global/activate/unknown_package_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 'package:scheduled_test/scheduled_test.dart';
 
 import '../../../lib/src/exit_codes.dart' as exit_codes;
-import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('errors if the package could not be found', () {
     servePackages([]);
 
     schedulePub(args: ["global", "activate", "foo"],
         error: startsWith("Could not find package foo at"),
         exitCode: exit_codes.UNAVAILABLE);
   });
 }