Add chain support to barback.

pkg/barback/lib/src/asset_node.dart

Index: pkg/barback/lib/src/asset_node.dart
diff --git a/pkg/barback/lib/src/asset_node.dart b/pkg/barback/lib/src/asset_node.dart
index 2372ba909c380217af965c26f882a64564c20acd..a1af547ddddc53cedd25b3f6044bf28cdbdad51d 100644
--- a/pkg/barback/lib/src/asset_node.dart
+++ b/pkg/barback/lib/src/asset_node.dart
@@ -10,6 +10,7 @@ import 'asset.dart';
 import 'asset_id.dart';
 import 'errors.dart';
 import 'transform_node.dart';
+import 'utils.dart';
 
 /// Describes the current state of an asset as part of a transformation graph.
 ///
@@ -125,7 +126,7 @@ class AssetNode {
   /// The return value of [callback] is piped to the returned Future.
   Future _waitForState(bool test(AssetState state),
       callback(AssetState state)) {
-    if (test(state)) return new Future.sync(() => callback(state));
+    if (test(state)) return syncFuture(() => callback(state));
     return onStateChange.firstWhere(test).then((_) => callback(state));
   }