fix barback and polymer build warnings

pkg/barback/lib/src/phase_input.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.phase_input;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 97 matching lines @@
     _onDirtyPool.close();
     _onLogPool.close();
     _inputForwarder.close();
     if (_passThroughController != null) {
       _passThroughController.setRemoved();
       _passThroughController = null;
     }
   }
 
   /// Set this input's transformers to [transformers].
-  void updateTransformers(Iterable<Transformer> newTransformers) {
-    newTransformers = newTransformers.toSet();
+  void updateTransformers(Iterable<Transformer> newTransformersIterable) {
+    var newTransformers = newTransformersIterable.toSet();
     var oldTransformers = _transformers.toSet();
     for (var removedTransformer in
          oldTransformers.difference(newTransformers)) {
       _transformers.remove(removedTransformer);
 
       // If the transformers are being adjusted for [id], it will
       // automatically pick up on [removedTransformer] being gone.
       if (_adjustTransformersFuture != null) continue;
 
       _transforms.removeWhere((transform) {
@@ skipping 176 matching lines @@
       return new Future.value(
           new Set<AssetNode>.from([_passThroughController.node]));
     }
 
     return Future.wait(_transforms.map((transform) {
       if (!transform.isDirty) return new Future.value(new Set());
       return transform.apply();
     })).then((outputs) => unionAll(outputs));
   }
 }