Support passing barback assets through phases in which they're unused.

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 'package:stack_trace/stack_trace.dart';
 
@@ skipping 67 matching lines @@
             _cascadeResults.values.map((result) => result.errors))));
       }, onError: _resultsController.addError);
     }
 
     _errors = mergeStreams(_cascades.values.map((cascade) => cascade.errors));
   }
 
   /// 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. If the asset cannot be found, returns
-  /// null.
+  /// 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) {
     var cascade = _cascades[id.package];
     if (cascade != null) return cascade.getAssetNode(id);
     return new Future.value(null);
   }
 
   /// Adds [sources] to the graph's known set of source assets.
   ///
   /// Begins applying any transforms that can consume any of the sources. If a
   /// given source is already known, it is considered modified and all
@@ skipping 10 matching lines @@
   /// Removes [removed] from the graph's known set of source assets.
   void removeSources(Iterable<AssetId> sources) {
     groupBy(sources, (id) => id.package).forEach((package, ids) {
       var cascade = _cascades[package];
       if (cascade == null) throw new ArgumentError("Unknown package $package.");
       _cascadeResults[package] = null;
       cascade.removeSources(ids);
     });
   }
 }