Add location information to the JSON reporter.

lib/src/backend/group_entry.dart

 // Copyright (c) 2015, 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:stack_trace/stack_trace.dart';
+
 import 'metadata.dart';
 import 'operating_system.dart';
 import 'test.dart';
 import 'test_platform.dart';
 
 /// A [Test] or [Group].
 abstract class GroupEntry {
   /// The name of the entry, including the prefixes from any containing
   /// [Group]s.
   ///
   /// This will be `null` for the root group.
   String get name;
 
   /// The metadata for the entry, including the metadata from any containing
   /// [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
   /// match [platform] and [selector]. Returns `null` if this entry's selector
   /// doesn't match.
   GroupEntry forPlatform(TestPlatform platform, {OperatingSystem os});
 
   /// Returns a copy of [this] with all tests that don't match [callback]
   /// removed.
   ///
   /// Returns `null` if this is a test that doesn't match [callback] or a group
   /// where no child tests match [callback].
   GroupEntry filter(bool callback(Test test));
 }