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

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 'asset_forwarder.dart';
 import 'asset_node.dart';
@@ skipping 32 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 => _onAssetPool.stream;
-  final _onAssetPool = new StreamPool<AssetNode>();
+  final _onAssetPool = new StreamPool<AssetNode>.broadcast();
 
   /// Whether [this] is dirty and still has more processing to do.
   bool get isDirty => _transforms.any((transform) => transform.isDirty);
 
   /// A stream that emits an event whenever any transforms that use [input] as
   /// their primary input log an entry.
   Stream<LogEntry> get onLog => _onLogPool.stream;
   final _onLogPool = new StreamPool<LogEntry>.broadcast();
 
   PhaseInput(this._phase, AssetNode input, this._location)
@@ skipping 38 matching lines @@
   /// Force all [LazyTransformer]s' transforms in this input to begin producing
   /// concrete assets.
   void forceAllTransforms() {
     for (var transform in _transforms) {
       transform.force();
     }
   }
 
   String toString() => "phase input in $_location for $input";
 }