Include file path in JSON error when possible.

sdk/lib/_internal/pub/test/global/activate/same_version_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 '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('keeps previously activated version if it meets the constraint',
         () {
     servePackages([
       packageMap("foo", "1.2.3"),
       packageMap("foo", "1.3.0")
     ]);
 
     // Activate 1.2.3.
     schedulePub(args: ["global", "activate", "foo", "1.2.3"]);
 
     // Activating it again re-resolves but maintains the version.
     schedulePub(args: ["global", "activate", "foo", ">1.0.0"], output: """
 Package foo is already active at version 1.2.3.
 Resolving dependencies...
 Activated foo 1.2.3.""");
   });
 }