Fix a barback bug where a result could be emitted incorrectly.

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 'asset_cascade.dart';
 import 'asset_id.dart';
@@ skipping 51 matching lines @@
 
   /// Creates a new [PackageGraph] that will transform assets in all packages
   /// made available by [provider].
   PackageGraph(this.provider) {
     for (var package in provider.packages) {
       var cascade = new AssetCascade(this, package);
       // The initial result for each cascade is "success" since the cascade
       // doesn't start building until some source in that graph is updated.
       _cascadeResults[package] = new BuildResult.success();
       _cascades[package] = cascade;
+      cascade.onDirty.listen((_) {
+        _cascadeResults[package] = null;
+      });
 
       cascade.results.listen((result) {
         _cascadeResults[cascade.package] = result;
         // If any cascade hasn't yet finished, the overall build isn't finished
         // either.
         if (_cascadeResults.values.any((result) => result == null)) return;
 
         // Include all build errors for all cascades. If no cascades have
         // errors, the result will automatically be considered a success.
         _resultsController.add(new BuildResult(unionAll(
@@ skipping 55 matching lines @@
 
   /// 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);
     });
   }
 
   void updateTransformers(String package,
       Iterable<Iterable<Transformer>> transformers) {
-    _cascadeResults[package] = null;
     _cascades[package].updateTransformers(transformers);
   }
 }