Fix all strong-mode warnings.

lib/src/transformer/wrapping_aggregate_transformer.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.
 
 library barback.transformer.wrapping_aggregate_transformer;
 
 import 'dart:async';
 
 import '../asset/asset_id.dart';
-import '../utils.dart';
 import 'aggregate_transform.dart';
 import 'aggregate_transformer.dart';
 import 'declaring_aggregate_transform.dart';
 import 'declaring_aggregate_transformer.dart';
 import 'declaring_transform.dart';
 import 'declaring_transformer.dart';
 import 'lazy_aggregate_transformer.dart';
 import 'lazy_transformer.dart';
 import 'transform.dart';
 import 'transformer.dart';
@@ skipping 16 matching lines @@
       return new _DeclaringWrappingAggregateTransformer(
           transformer as DeclaringTransformer);
     } else {
       return new WrappingAggregateTransformer._(transformer);
     }
   }
 
   WrappingAggregateTransformer._(this.transformer);
 
   Future<String> classifyPrimary(AssetId id) {
-    return syncFuture(() => transformer.isPrimary(id))
+    return new Future.sync(() => transformer.isPrimary(id))
         .then((isPrimary) => isPrimary ? id.path : null);
   }
 
   Future apply(AggregateTransform aggregateTransform) {
     return newTransform(aggregateTransform)
         .then((transform) => transformer.apply(transform));
   }
 
   String toString() => transformer.toString();
 }
@@ skipping 17 matching lines @@
 }
 
 /// A wrapper for [LazyTransformer]s that implements
 /// [LazyAggregateTransformer].
 class _LazyWrappingAggregateTransformer
     extends _DeclaringWrappingAggregateTransformer
     implements LazyAggregateTransformer {
   _LazyWrappingAggregateTransformer(LazyTransformer transformer)
       : super(transformer);
 }