fix barback and polymer build warnings

pkg/barback/lib/src/asset_cascade.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 barback.asset_cascade;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 194 matching lines @@
       var controller = _sourceControllerMap.remove(id);
       // Don't choke if an id is double-removed for some reason.
       if (controller != null) controller.setRemoved();
     });
   }
 
   /// Sets this cascade's transformer phases to [transformers].
   ///
   /// Elements of the inner iterable of [transformers] must be either
   /// [Transformer]s or [TransformerGroup]s.
-  void updateTransformers(Iterable<Iterable> transformers) {
-    transformers = transformers.toList();
+  void updateTransformers(Iterable<Iterable> transformersIterable) {
+    var transformers = transformersIterable.toList();
 
     for (var i = 0; i < transformers.length; i++) {
       if (_phases.length > i) {
         _phases[i].updateTransformers(transformers[i]);
         continue;
       }
 
       _addPhase(_phases.last.addPhase(transformers[i]));
     }
 
@@ skipping 66 matching lines @@
         future = phase.process();
         if (future != null) break;
       }
 
       // If all phases are done and no new updates have come in, we're done.
       if (future == null) {
         // If changes have come in, start over.
         if (_newChanges) return _process();
 
         // Otherwise, everything is done.
-        return;
+        return null;
       }
 
       // Process that phase and then loop onto the next.
       return future.then((_) => _process());
     });
   }
 }