Add more metadata to non-programmatic barback exceptions.

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 34 matching lines @@
   Stream<BuildResult> get results => _resultsController.stream;
   final _resultsController = new StreamController<BuildResult>.broadcast();
 
   /// A stream that emits any errors from the graph or the transformers.
   ///
   /// This emits errors as they're detected. If an error occurs in one part of
   /// the graph, unrelated parts will continue building.
   ///
   /// This will not emit programming errors from barback itself. Those will be
   /// emitted through the [results] stream's error channel.
-  Stream get errors => _errors;
-  Stream _errors;
+  Stream<BarbackException> get errors => _errors;
+  Stream<BarbackException> _errors;
 
   /// 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,
           provider.getTransformers(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.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(flatten(
+        _resultsController.add(new BuildResult(unionAll(
             _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
@@ skipping 22 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);
     });
   }
 }