fix barback and polymer build warnings

pkg/scheduled_test/lib/scheduled_process.dart

Index: pkg/scheduled_test/lib/scheduled_process.dart
diff --git a/pkg/scheduled_test/lib/scheduled_process.dart b/pkg/scheduled_test/lib/scheduled_process.dart
index 50ff5f721dfbd292a30e5557f33dfb473090bb20..8f91459a70e8d590a1b7153271f69309e4f87120 100644
--- a/pkg/scheduled_test/lib/scheduled_process.dart
+++ b/pkg/scheduled_test/lib/scheduled_process.dart
@@ -176,7 +176,7 @@ class ScheduledProcess {
       }
 
       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.
@@ -216,7 +216,7 @@ class ScheduledProcess {
       _stdoutCanceller();
       _stderrCanceller();
 
-      if (!_process.hasValue) return;
+      if (!_process.hasValue) return null;
 
       var killedPrematurely = false;
       if (!_exitCode.hasValue) {