Include file path in JSON error when possible.

sdk/lib/_internal/pub/test/cache/repair/handles_failure_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.
 
 library pub_tests;
 
 import 'package:scheduled_test/scheduled_test.dart';
-import 'package:scheduled_test/scheduled_stream.dart';
 
 import '../../../lib/src/exit_codes.dart' as exit_codes;
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('handles failure to reinstall some packages', () {
     // Only serve two packages so repairing will have a failure.
     servePackages([
@@ skipping 30 matching lines @@
 
     pub.stderr.expect(startsWith("Failed to repair foo 1.2.4. Error:"));
     pub.stderr.expect("HTTP error 404: Not Found");
 
     pub.stdout.expect("Reinstalled 2 packages.");
     pub.stdout.expect("Failed to reinstall 1 package.");
 
     pub.shouldExit(exit_codes.UNAVAILABLE);
   });
 }