fix barback and polymer build warnings

pkg/barback/test/utils.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.test.utils;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert' show Encoding;
 
@@ skipping 448 matching lines @@
     // simulates the real world, where there'll always be at least the
     // entrypoint package.
     if (_assets.isEmpty) {
       _assets = {"app": new AssetSet()};
     }
   }
 
   void _modifyAsset(String name, String contents) {
     var id = new AssetId.parse(name);
     _errors.remove(id);
-    _assets[id.package][id].contents = contents;
+    (_assets[id.package][id] as _MockAsset).contents = contents;
   }
 
   void _setAssetError(String name) {
     _errors.add(new AssetId.parse(name));
   }
 
   List<AssetId> listAssets(String package, {String within}) {
     if (within != null) {
       throw new UnimplementedError("Doesn't handle 'within' yet.");
     }
@@ skipping 41 matching lines @@
 
   _MockAsset(this.id, this.contents);
 
   Future<String> readAsString({Encoding encoding}) =>
       new Future.value(contents);
 
   Stream<List<int>> read() => throw new UnimplementedError();
 
   String toString() => "MockAsset $id $contents";
 }