Include file path in JSON error when possible.

sdk/lib/_internal/pub/test/global/activate/different_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("discards the previous active version if it doesn't match the "
       "constraint", () {
     servePackages([
       packageMap("foo", "1.0.0"),
       packageMap("foo", "2.0.0")
     ]);
 
     // Activate 1.0.0.
     schedulePub(args: ["global", "activate", "foo", "1.0.0"]);
 
     // Activating it again with a different constraint changes the version.
     schedulePub(args: ["global", "activate", "foo", ">1.0.0"], output: """
 Package foo is already active at version 1.0.0.
 Downloading foo 2.0.0...
 Resolving dependencies...
 Activated foo 2.0.0.""");
   });
 }