Make pkg/stack_trace use stack chains.

pkg/scheduled_test/test/scheduled_process_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.
 
 library scheduled_process_test;
 
 import 'dart:async';
-import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 import 'package:scheduled_test/scheduled_process.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import 'metatest.dart';
 import 'utils.dart';
 
 void main(_, message) {
@@ skipping 365 matching lines @@
   currentSchedule.onComplete.schedule(() {
     return tempDir.catchError((_) => null).then((dir) {
       if (dir == null) return null;
       return new Directory(dir).delete(recursive: true);
     });
   }, 'clean up temp dir');
 
   return new ScheduledProcess.start(Platform.executable,
       ['--checked', dartPath]);
 }