Add location information to the JSON reporter.

lib/src/backend/test.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 'group.dart';
 import 'group_entry.dart';
 import 'live_test.dart';
 import 'metadata.dart';
 import 'operating_system.dart';
 import 'suite.dart';
 import 'test_platform.dart';
 
 /// A single test.
 ///
 /// A test is immutable and stateless, which means that it can't be run
 /// directly. To run one, load a live version using [Test.load] and run it using
 /// [LiveTest.run].
 abstract class Test implements GroupEntry {
   String get name;
 
   Metadata get metadata;
 
+  Trace get trace;
+
   /// Loads a live version of this test, which can be used to run it a single
   /// time.
   ///
   /// [suite] is the suite within which this test is being run. If [groups] is
   /// passed, it's the list of groups containing this test; otherwise, it
   /// defaults to just containing `suite.group`.
   LiveTest load(Suite suite, {Iterable<Group> groups});
 
   Test forPlatform(TestPlatform platform, {OperatingSystem os});
 
   Test filter(bool callback(Test test)) => callback(this) ? this : null;
 }