fix barback and polymer build warnings

pkg/scheduled_test/lib/scheduled_process.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_test.scheduled_process;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 158 matching lines @@
     // process is running, we want the schedule to move to the onException
     // queue where the process will be killed, rather than blocking the tasks
     // queue waiting for the process to exit.
     _process.then((p) => p.exitCode).then((exitCode) {
       if (_endExpected) {
         exitCodeCompleter.complete(exitCode);
         return;
       }
 
       wrapFuture(pumpEventQueue().then((_) {
-        if (currentSchedule.currentTask != _taskBeforeEnd) return;
+        if (currentSchedule.currentTask != _taskBeforeEnd) return null;
         // If we're one task before the end was scheduled, wait for that task
         // to complete and pump the event queue so that _endExpected will be
         // set.
         return _taskBeforeEnd.result.then((_) => pumpEventQueue());
       }).then((_) {
         exitCodeCompleter.complete(exitCode);
 
         if (!_endExpected) {
           fail("Process '$description' ended earlier than scheduled "
                "with exit code $exitCode.");
@@ skipping 19 matching lines @@
         .transform(new LineSplitter()));
   }
 
   /// Schedule an exception handler that will clean up the process and provide
   /// debug information if an error occurs.
   void _scheduleExceptionCleanup() {
     currentSchedule.onException.schedule(() {
       _stdoutCanceller();
       _stderrCanceller();
 
-      if (!_process.hasValue) return;
+      if (!_process.hasValue) return null;
 
       var killedPrematurely = false;
       if (!_exitCode.hasValue) {
         killedPrematurely = true;
         _endExpected = true;
         _process.value.kill(ProcessSignal.SIGKILL);
         // Ensure that the onException queue waits for the process to actually
         // exit after being killed.
         wrapFuture(_process.value.exitCode, "waiting for process "
             "'$description' to die");
@@ skipping 118 matching lines @@
     schedule(() {
       _endExpected = true;
       return _exitCode.then((exitCode) {
         if (expectedExitCode != null) {
           expect(exitCode, equals(expectedExitCode));
         }
       });
     }, "waiting for process '$description' to exit");
   }
 }