pkg/barback: removed unused imports, added a missing import

pkg/barback/lib/src/phase.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;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
 import 'asset_cascade.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
-import 'asset_set.dart';
 import 'errors.dart';
 import 'stream_pool.dart';
 import 'transform_node.dart';
 import 'transformer.dart';
 import 'utils.dart';
 
 /// One phase in the ordered series of transformations in an [AssetCascade].
 ///
 /// Each phase can access outputs from previous phases and can in turn pass
 /// outputs to later phases. Phases are processed strictly serially. All
@@ skipping 291 matching lines @@
   }
 
   Future _waitForInputs() {
     if (_adjustTransformersFutures.isEmpty) return new Future.value();
     return Future.wait(_adjustTransformersFutures.values)
         .then((_) => _waitForInputs());
   }
 
   /// Applies all currently wired up and dirty transforms.
   Future _processTransforms() {
-    if (_next == null) return;
+    if (_next == null) return new Future.value();

[Bob Nystrom, 2013/08/19 23:03:55]

This will go away when you sync to latest code, I think.

 
     var newPassThroughs = _passThroughControllers.values
         .map((controller) => controller.node)
         .where((output) {
       return !_outputs.containsKey(output.id) ||
         !_outputs[output.id].contains(output);
     }).toSet();
 
     // Convert this to a list so we can safely modify _transforms while
     // iterating over it.
@@ skipping 87 matching lines @@
         // Pump the event queue to ensure that the removal of the input triggers
         // a new build to which we can attach the error.
         newFuture(() => cascade.reportError(new AssetCollisionException(
             assets.where((asset) => asset.transform != null)
                 .map((asset) => asset.transform.info),
             output.id)));
       }
     });
   }
 }