Support cross-package transforms in 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 'package:stack_trace/stack_trace.dart';
 
 import 'asset.dart';
 import 'asset_cascade.dart';
 import 'asset_id.dart';
+import 'asset_node.dart';
+import 'build_result.dart';
 import 'errors.dart';
 import 'package_provider.dart';
 import 'utils.dart';
 
 /// The collection of [AssetCascade]s for an entire application.
 ///
 /// This tracks each package's [AssetCascade] and routes asset requests between
 /// them.
 class PackageGraph {
   /// The provider that exposes asset and package information.
@@ skipping 49 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(flatten(
             _cascadeResults.values.map((result) => result.errors))));
       }, onError: _resultsController.addError);
     }
 
     _errors = mergeStreams(_cascades.values.map((cascade) => cascade.errors));
   }
 
-  /// Gets the asset identified by [id].
+  /// 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, throws
-  /// [AssetNotFoundException].
-  Future<Asset> getAssetById(AssetId 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.
+  Future<AssetNode> getAssetNode(AssetId id) {
     var cascade = _cascades[id.package];
-    if (cascade != null) return cascade.getAssetById(id);
-    return new Future.error(
-        new AssetNotFoundException(id),
-        new Trace.current().vmTrace);
+    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
   /// transforms that use it will be re-applied.
   void updateSources(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.updateSources(ids);
     });
   }
 
   /// 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);
     });
   }
 }