Fix a synchrony bug in 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 257 matching lines @@
       return callback(asset, _transformers);
     }).then((result) {
       if (setEquals(oldTransformers, _transformers)) return result;
       return _tryUntilStable(callback);
     });
   }
 
   /// Processes the transforms for this input.
   Future<Set<AssetNode>> process() {
     if (_adjustTransformersFuture == null) return _processTransforms();
-    return _waitForInputs().then((_) => _processTransforms());
+    return _waitForTransformers(() => _processTransforms());
   }
 
-  Future _waitForInputs() {
-    // Return a synchronous future so we can be sure [_adjustTransformers] isn't
-    // called between now and when the Future completes.
-    if (_adjustTransformersFuture == null) return new Future.sync(() {});
-    return _adjustTransformersFuture.then((_) => _waitForInputs());
+  /// 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);
+    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));
   }
 }