Add the ability for a PackageProvider to declare static packages.

pkg/barback/lib/src/graph/asset_cascade.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.graph.asset_cascade;
 
 import 'dart:async';
 
 import '../asset/asset.dart';
 import '../asset/asset_id.dart';
@@ skipping 64 matching lines @@
     // itself.
     return _phases.last.status;
   }
 
   /// The streams exposed by this cascade.
   final _streams = new NodeStreams();
   Stream<LogEntry> get onLog => _streams.onLog;
   Stream<NodeStatus> get onStatusChange => _streams.onStatusChange;
 
   /// Returns all currently-available output assets from this cascade.
-  AssetSet get availableOutputs =>
-    new AssetSet.from(_phases.last.availableOutputs.map((node) => node.asset));
+  Future<AssetSet> get availableOutputs => new Future.value(new AssetSet.from(
+      _phases.last.availableOutputs.map((node) => node.asset)));
 
   /// Creates a new [AssetCascade].
   ///
   /// It loads source assets within [package] using [provider].
   AssetCascade(this.graph, this.package) {
     _addPhase(new Phase(this, package));
   }
 
   /// Gets the asset identified by [id].
   ///
@@ skipping 117 matching lines @@
     _streams.onLogPool.add(phase.onLog);
     if (_phaseStatusSubscription != null) _phaseStatusSubscription.cancel();
     _phaseStatusSubscription =
         phase.onStatusChange.listen(_streams.changeStatus);
 
     _phases.add(phase);
   }
 
   String toString() => "cascade for $package";
 }