Add an AssetStream class to Barback.

pkg/barback/lib/src/package_graph.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.package_graph;
 
 import 'dart:async';
 
 import 'asset_cascade.dart';
 import 'asset_id.dart';
@@ skipping 68 matching lines @@
         // Include all build errors for all cascades. If no cascades have
         // errors, the result will automatically be considered a success.
         _resultsController.add(new BuildResult(unionAll(
             _cascadeResults.values.map((result) => result.errors))));
       }, onError: (error) {
         _lastUnexpectedError = error;
         _resultsController.addError(error);
       });
     }
 
-    _errors = mergeStreams(_cascades.values.map((cascade) => cascade.errors));
+    _errors = mergeStreams(_cascades.values.map((cascade) => cascade.errors),
+        broadcast: true);
   }
 
   /// Gets the asset node identified by [id].
   ///
   /// If [id] is for a generated or transformed asset, this will wait until it
   /// has been created and return it. This means that the returned asset will
   /// always be [AssetState.AVAILABLE].
   ///
   /// If the asset cannot be found, returns null.
   Future<AssetNode> getAssetNode(AssetId id) {
@@ skipping 57 matching lines @@
       cascade.removeSources(ids);
     });
   }
 
   void updateTransformers(String package,
       Iterable<Iterable<Transformer>> transformers) {
     _cascadeResults[package] = null;
     _cascades[package].updateTransformers(transformers);
   }
 }