Add location information to the JSON reporter.

lib/src/backend/group_entry.dart

Index: lib/src/backend/group_entry.dart
diff --git a/lib/src/backend/group_entry.dart b/lib/src/backend/group_entry.dart
index b9fb7c6f3f260e058c40ad52c054b31e85ba2e4b..bde401eeab5c594fd456237ee805d382c16f9695 100644
--- a/lib/src/backend/group_entry.dart
+++ b/lib/src/backend/group_entry.dart
@@ -2,6 +2,8 @@
 // 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:stack_trace/stack_trace.dart';
+
 import 'metadata.dart';
 import 'operating_system.dart';
 import 'test.dart';
@@ -19,6 +21,10 @@ abstract class GroupEntry {
   /// [Group]s.
   Metadata get metadata;
 
+  /// The stack trace for the call to `test()` or `group()` that defined this
+  /// entry, or `null` if the entry was defined in a different way.
+  Trace get trace;
+
   /// Returns a copy of [this] with all platform-specific metadata resolved.
   ///
   /// Removes any tests and groups with [Metadata.testOn] selectors that don't