Add location information to the JSON reporter.

lib/src/backend/test.dart

Index: lib/src/backend/test.dart
diff --git a/lib/src/backend/test.dart b/lib/src/backend/test.dart
index 5d3ac5372e88611e3a6fd4e4107d39697b616b60..62b12aea945c1fb95576d7ba3d940ecaf6a50d3a 100644
--- a/lib/src/backend/test.dart
+++ b/lib/src/backend/test.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 'group.dart';
 import 'group_entry.dart';
 import 'live_test.dart';
@@ -20,6 +22,8 @@ abstract class Test implements GroupEntry {
 
   Metadata get metadata;
 
+  Trace get trace;
+
   /// Loads a live version of this test, which can be used to run it a single
   /// time.
   ///