Include file path in JSON error when possible.

sdk/lib/_internal/pub/test/global/activate/installs_dependencies_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('activating a package installs its dependencies too', () {
     servePackages([
       packageMap("foo", "1.0.0", {"bar": "any"}),
       packageMap("bar", "1.0.0", {"baz": "any"}),
       packageMap("baz", "1.0.0")
     ]);
 
     schedulePub(args: ["global", "activate", "foo"], output: allOf([
       contains("Downloading bar 1.0.0..."),
       contains("Downloading baz 1.0.0...")
     ]));
   });
 }