Include file path in JSON error when possible.

sdk/lib/_internal/pub/test/build/preserves_htaccess_test.dart

 // Copyright (c) 2013, 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("preserves .htaccess as a special case", () {
     d.dir(appPath, [
       d.appPubspec(),
       d.dir('web', [
         d.file('.htaccess', 'fblthp'),
         d.file('.hidden', 'asdfgh')
       ])
     ]).create();
 
     schedulePub(args: ["build"],
         output: new RegExp(r'Built \d+ files? to "build".'));
 
     d.dir(appPath, [
       d.dir('build', [
         d.dir('web', [
           d.file('.htaccess', 'fblthp'),
           d.nothing('.hidden')
         ])
       ])
     ]).validate();
   });
 }