Include file path in JSON error when possible.

sdk/lib/_internal/pub/test/implicit_dependency_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';
 import '../lib/src/barback.dart' as barback;
 import '../lib/src/version.dart';
 
 main() {
   initConfig();
 
   forBothPubGetAndUpgrade((command) {
     integration("implicitly constrains it to versions pub supports", () {
@@ skipping 100 matching lines @@
   var constraint = barback.pubConstraints[packageName];
   return new Version(constraint.min.major, constraint.min.minor - 1, 0)
       .toString();
 }
 
 String nextPatch(String packageName) =>
     barback.pubConstraints[packageName].min.nextPatch.toString();
 
 String max(String packageName) =>
     barback.pubConstraints[packageName].max.toString();