fix barback and polymer build warnings

pkg/scheduled_test/lib/src/task.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 task;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:stack_trace/stack_trace.dart';
@@ skipping 62 matching lines @@
       : queue = queue,
         stackTrace = new Trace.current() {
     this.fn = () {
       if (state != TaskState.WAITING) {
         throw new StateError("Can't run $state task '$this'.");
       }
 
       _state = TaskState.RUNNING;
       var future = new Future.value().then((_) => fn())
           .whenComplete(() {
-        if (_childGroup == null || _childGroup.completed) return;
+        if (_childGroup == null || _childGroup.completed) return null;
         return _childGroup.future;
       });
       chainToCompleter(future, _resultCompleter);
       return future;
     };
 
     // If the parent queue experiences an error before this task has started
     // running, pipe that error out through [result]. This ensures that we don't
     // get deadlocked by something like `expect(schedule(...), completes)`.
     queue.onTasksComplete.catchError((e) {
@@ skipping 59 matching lines @@
   final String name;
 
   /// Whether the state indicates that the task has finished running. This is
   /// true for both the [SUCCESS] and [ERROR] states.
   bool get isDone => this == SUCCESS || this == ERROR;
 
   const TaskState._(this.name);
 
   String toString() => name;
 }