Fix all strong-mode warnings.

lib/src/transformer/wrapping_aggregate_transformer.dart

Index: lib/src/transformer/wrapping_aggregate_transformer.dart
diff --git a/lib/src/transformer/wrapping_aggregate_transformer.dart b/lib/src/transformer/wrapping_aggregate_transformer.dart
index f41d07584a3fdcd742e6bfcd89c52081ad02518d..a24260da3f4acb79e0d2ca6b704c596f4ce528a1 100644
--- a/lib/src/transformer/wrapping_aggregate_transformer.dart
+++ b/lib/src/transformer/wrapping_aggregate_transformer.dart
@@ -7,7 +7,6 @@ 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';
@@ -44,7 +43,7 @@ class WrappingAggregateTransformer implements AggregateTransformer {
   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);
   }