fix barback and polymer build warnings

pkg/scheduled_test/lib/src/schedule.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 schedule;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:stack_trace/stack_trace.dart';
@@ skipping 421 matching lines @@
     return task.result;
   }
 
   /// Runs all the tasks in this queue in order.
   Future _run() {
     _schedule._currentQueue = this;
     _schedule.heartbeat();
     return Future.forEach(_contents, (task) {
       _schedule._currentTask = task;
       if (_error != null) throw _error;
-      if (_aborted) return;
+      if (_aborted) return null;
 
       _taskFuture = new SubstituteFuture(task.fn());
       return _taskFuture.whenComplete(() {
         _taskFuture = null;
         _schedule.heartbeat();
       }).catchError((e, trace) {
         var error = new ScheduleError.from(_schedule, e, stackTrace: trace);
         _signalError(error);
         throw _error;
       });
     }).whenComplete(() {
       _schedule._currentTask = null;
     }).then((_) {
       _onTasksCompleteCompleter.complete();
     }).catchError((e, stackTrace) {
       _onTasksCompleteCompleter.completeError(e, stackTrace);
       throw e;
     }).whenComplete(() {
-      if (pendingCallbacks.isEmpty) return;
+      if (pendingCallbacks.isEmpty) return null;
       return _noPendingCallbacks.catchError((e, stackTrace) {
         // Signal the error rather than passing it through directly so that if a
         // timeout happens after an in-task error, both are reported.
         _signalError(new ScheduleError.from(_schedule, e,
                                             stackTrace: stackTrace));
       });
     }).whenComplete(() {
       _schedule.heartbeat();
       // If the tasks were otherwise successful, make sure we throw any
       // out-of-band errors. If a task failed, make sure we throw the most
@@ skipping 39 matching lines @@
         return fn(arg);
       } catch (e, stackTrace) {
         var error = new ScheduleError.from(
             _schedule, e, stackTrace: stackTrace);
         if (_timedOut()) {
           _schedule._signalPostTimeoutError(error);
         } else {
           _schedule.signalError(error);
         }
       } finally {
-        if (_timedOut()) return;
+        if (_timedOut()) return null;
 
         _pendingCallbacks.remove(pendingCallback);
         if (_pendingCallbacks.isEmpty && !isRunningTasks) {
           _noPendingCallbacksCompleter.complete();
         }
       }
     };
   }
 
   /// Signals that an out-of-band error has been detected and the queue should
@@ skipping 74 matching lines @@
   /// The string description of the callback.
   String get description {
     if (_description == null) _description = _thunk();
     return _description;
   }
 
   String toString() => description;
 
   PendingCallback._(this._thunk);
 }