switch package to strong mode

test/resolver_test.dart

 // Copyright (c) 2014, 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.
 
 @TestOn('vm')
 library code_transformers.test.resolver_test;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
@@ skipping 415 matching lines @@
   final bool resolveAllLibraries;
 
   TestTransformer(Resolvers resolvers, this.primary, this.validator,
       this.resolveAllLibraries) {
     this.resolvers = resolvers;
   }
 
   // TODO(nweiz): This should just take an AssetId when barback <0.13.0 support
   // is dropped.
   Future<bool> isPrimary(idOrAsset) {
-    var id = idOrAsset is AssetId ? idOrAsset : idOrAsset.id;
+    var id = idOrAsset is AssetId ? idOrAsset : (idOrAsset as Asset).id;
     return new Future.value(id == primary);
   }
 
   applyResolver(Transform transform, Resolver resolver) => validator(resolver);
 }