fix barback and polymer build warnings

pkg/barback/lib/src/asset_cascade.dart

Index: pkg/barback/lib/src/asset_cascade.dart
diff --git a/pkg/barback/lib/src/asset_cascade.dart b/pkg/barback/lib/src/asset_cascade.dart
index cc9056012be4b7c2dd79ef0ca941970f889ef24e..ac57600ebcfc61960e641bb2371fc20f6f9f647d 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -212,8 +212,8 @@ class AssetCascade {
   ///
   /// 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) {
@@ -300,7 +300,7 @@ class AssetCascade {
         if (_newChanges) return _process();
 
         // Otherwise, everything is done.
-        return;
+        return null;
       }
 
       // Process that phase and then loop onto the next.