Add chain support to barback.

pkg/barback/lib/src/phase_input.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.phase_input;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 273 matching lines @@
     });
   }
 
   /// Runs [callback] once all the transformers are adjusted correctly and the
   /// input is ready to be processed.
   ///
   /// If the transformers are already properly adjusted, [callback] is called
   /// synchronously to ensure that [_adjustTransformers] isn't called before the
   /// callback.
   Future _waitForTransformers(callback()) {
-    if (_adjustTransformersFuture == null) return new Future.sync(callback);
+    if (_adjustTransformersFuture == null) return syncFuture(callback);
     return _adjustTransformersFuture.then(
         (_) => _waitForTransformers(callback));
   }
 
   /// Applies all currently wired up and dirty transforms.
   Future<Set<AssetNode>> _processTransforms() {
     if (input.state.isRemoved) return new Future.value(new Set());
 
     if (_passThroughController != null) {
       if (!_newPassThrough) return new Future.value(new Set());
       _newPassThrough = false;
       return new Future.value(
           new Set<AssetNode>.from([_passThroughController.node]));
     }
 
     return Future.wait(_transforms.map((transform) {
       if (!transform.isDirty) return new Future.value(new Set());
       return transform.apply();
     })).then((outputs) => unionAll(outputs));
   }
 }