Fix a synchrony bug in barback.

pkg/barback/lib/src/transform_node.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.transform_node;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 142 matching lines @@
   /// [AssetNotFoundException].
   Future<Asset> getInput(AssetId id) {
     return phase.getInput(id).then((node) {
       // Throw if the input isn't found. This ensures the transformer's apply
       // is exited. We'll then catch this and report it through the proper
       // results stream.
       if (node == null) throw new MissingInputException(info, id);
 
       // If the asset node is found, wait until its contents are actually
       // available before we return them.
-      return node.whenAvailable.then((asset) {
+      return node.whenAvailable((asset) {
         _inputSubscriptions.putIfAbsent(node.id,
             () => node.onStateChange.listen((_) => _dirty()));
 
         return asset;
       }).catchError((error) {
         if (error is! AssetNotFoundException || error.id != id) throw error;
         // If the node was removed before it could be loaded, treat it as though
         // it never existed and throw a MissingInputException.
         throw new MissingInputException(info, id);
       });
@@ skipping 37 matching lines @@
   }
 
   void _log(AssetId asset, LogLevel level, String message, Span span) {
     // If the log isn't already associated with an asset, use the primary.
     if (asset == null) asset = primary.id;
     var info = new TransformInfo(transformer, primary.id);
     var entry = new LogEntry(info, asset, level, message, span);
     _onLogController.add(entry);
   }
 }