Move Barback to a more thoroughly push-based model.

pkg/barback/lib/src/phase_output.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_output;
 
 import 'dart:async';
 import 'dart:collection';
 
-import 'asset_cascade.dart';
 import 'asset_forwarder.dart';
 import 'asset_node.dart';
 import 'errors.dart';
 import 'phase.dart';
-import 'utils.dart';
 
 /// A class that handles a single output of a phase.
 ///
 /// Normally there's only a single [AssetNode] for a phase's output, but it's
 /// possible that multiple transformers in the same phase emit assets with the
 /// same id, causing collisions. This handles those collisions by forwarding the
 /// chronologically first asset.
 ///
 /// When the asset being forwarding changes, the old value of [output] will be
 /// marked as removed and a new value will replace it. Users of this class can
 /// be notified of this using [onAsset].
 class PhaseOutput {
   /// The phase for which this is an output.
   final Phase _phase;
 
   /// A string describing the location of [this] in the transformer graph.
   final String _location;
 
   /// The asset node for this output.
   AssetNode get output => _outputForwarder.node;
   AssetForwarder _outputForwarder;
 
   /// A stream that emits an [AssetNode] each time this output starts forwarding
   /// a new asset.
   Stream<AssetNode> get onAsset => _onAssetController.stream;
-  final _onAssetController = new StreamController<AssetNode>();
+  final _onAssetController = new StreamController<AssetNode>(sync: true);
 
   /// The assets for this output.
   ///
   /// If there's no collision, this will only have one element. Otherwise, it
   /// will be ordered by which asset was added first.
   final _assets = new Queue<AssetNode>();
 
   /// The [AssetCollisionException] for this output, or null if there is no
   /// collision currently.
   AssetCollisionException get collisionException {
@@ skipping 46 matching lines @@
       var wasFirst = _assets.first == node;
       _assets.remove(node);
 
       // If this was the first asset, we replace it with the next asset
       // (chronologically).
       if (wasFirst) removeListeners();
 
       // If there's still a collision, report it. This lets the user know if
       // they've successfully resolved the collision or not.
       if (_assets.length > 1) {
-        // Pump the event queue to ensure that the removal of the input triggers
-        // a new build to which we can attach the error.
         // TODO(nweiz): report this through the output asset.
-        newFuture(() => _phase.cascade.reportError(collisionException));
+        _phase.cascade.reportError(collisionException);
       }
     });
   }
 
   String toString() => "phase output in $_location for $output";
 }