Make pkg/stack_trace use stack chains.

pkg/scheduled_test/lib/src/scheduled_server/handler.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_server.handler;
 
 import 'dart:async';
 
 import '../../scheduled_server.dart';
 import '../../scheduled_test.dart';
@@ skipping 35 matching lines @@
       return _waitForTask().then((_) {
         if (!ready) {
           fail("'${server.description}' received "
                "$method $path earlier than expected.");
         }
 
         // Use a nested call to [schedule] to help the user tell the difference
         // between a test failing while waiting for a handler and a test failing
         // while executing a handler.
         chainToCompleter(schedule(() {
-          return new Future.sync(() {
+          return syncFuture(() {
             if (request.method != method || request.uri.path != path) {
               fail("'${server.description}' expected $method $path, "
                    "but got ${request.method} ${request.uri.path}.");
             }
 
             return fn(request);
           });
         }, "'${server.description}' handling ${request.method} ${request.uri}"),
             _resultCompleter);
       });
     };
   }
 
   /// If the current task is [_taskBefore], waits for it to finish before
   /// completing. Otherwise, completes immediately.
   Future _waitForTask() {
     return pumpEventQueue().then((_) {
-      if (currentSchedule.currentTask != _taskBefore) return;
+      if (currentSchedule.currentTask != _taskBefore) return null;
       // If we're one task before the handler was scheduled, wait for that
       // task to complete and pump the event queue so that [ready] will be
       // set.
       return _taskBefore.result.then((_) => pumpEventQueue());
     });
   }
 }