Make barback's onAsset streams broadcast rather than single-subscriber.

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 55 matching lines @@
   /// This is synchronous in order to guarantee that it will emit an event as
   /// soon as [isDirty] flips from `true` to `false`.
   Stream get onDone => _onDoneController.stream;
   final _onDoneController = new StreamController.broadcast(sync: true);
 
   /// A stream that emits any new assets emitted by [this].
   ///
   /// Assets are emitted synchronously to ensure that any changes are thoroughly
   /// propagated as soon as they occur.
   Stream<AssetNode> get onAsset => _onAssetController.stream;
-  final _onAssetController = new StreamController<AssetNode>(sync: true);
+  final _onAssetController =
+      new StreamController<AssetNode>.broadcast(sync: true);
 
   /// A stream that emits an event whenever this transform logs an entry.
   ///
   /// This is synchronous because error logs can cause the transform to fail, so
   /// we need to ensure that their processing isn't delayed until after the
   /// transform or build has finished.
   Stream<LogEntry> get onLog => _onLogPool.stream;
   final _onLogPool = new StreamPool<LogEntry>.broadcast();
 
   /// The current state of [this].
@@ skipping 404 matching lines @@
   ///
   /// Specifically, whether [this] is [APPLIED] or [NOT_PRIMARY].
   bool get isDone => isApplied || isNotPrimary;
 
   final String name;
 
   const _TransformNodeState._(this.name);
 
   String toString() => name;
 }