Add chain support to 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 'asset_cascade.dart';
 import 'asset_id.dart';
@@ skipping 48 matching lines @@
   /// The stream of [LogEntry] objects used to report transformer log entries.
   Stream<LogEntry> get log => _logController.stream;
   final _logController = new StreamController<LogEntry>.broadcast();
 
   /// The most recent error emitted from a cascade's result stream.
   ///
   /// This is used to pipe an unexpected error from a build to the resulting
   /// [Future] returned by [getAllAssets].
   var _lastUnexpectedError;
 
+  /// The stack trace for [_lastUnexpectedError].
+  StackTrace _lastUnexpectedErrorTrace;
+
   // TODO(nweiz): Allow transformers to declare themselves as "lightweight" or
   // "heavyweight" and adjust their restrictions appropriately. Simple
   // transformers may be very efficient to run in parallel, whereas dart2js uses
   // a lot of memory and should be run more sequentially.
   /// A pool that controls how many transformers may be applied at once.
   final Pool transformPool = new Pool(10);
 
   /// Creates a new [PackageGraph] that will transform assets in all packages
   /// made available by [provider].
   PackageGraph(this.provider) {
@@ skipping 28 matching lines @@
       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.aggregate(_cascadeResults.values));
-      }, onError: (error, [stackTrace]) {
+      }, onError: (error, stackTrace) {
         _lastUnexpectedError = error;
+        _lastUnexpectedErrorTrace = stackTrace;
         _resultsController.addError(error, stackTrace);
       });
     }
 
     _errors = mergeStreams(_cascades.values.map((cascade) => cascade.errors),
         broadcast: true);
   }
 
   /// Gets the asset node identified by [id].
   ///
@@ skipping 16 matching lines @@
   Future<AssetSet> getAllAssets() {
     if (_cascadeResults.values.contains(null)) {
       // A build is still ongoing, so wait for it to complete and try again.
       return results.first.then((_) => getAllAssets());
     }
 
     // If an unexpected error occurred, complete with that.
     if (_lastUnexpectedError != null) {
       var error = _lastUnexpectedError;
       _lastUnexpectedError = null;
-      return new Future.error(error);
+      return new Future.error(error, _lastUnexpectedErrorTrace);
     }
 
     // If the build completed with an error, complete the future with it.
     var result = new BuildResult.aggregate(_cascadeResults.values);
     if (!result.succeeded) {
       return new Future.error(BarbackException.aggregate(result.errors));
     }
 
     // Otherwise, return all of the final output assets.
     var assets = unionAll(_cascades.values.map(
@@ skipping 22 matching lines @@
       if (cascade == null) throw new ArgumentError("Unknown package $package.");
       cascade.removeSources(ids);
     });
   }
 
   void updateTransformers(String package,
       Iterable<Iterable<Transformer>> transformers) {
     _cascades[package].updateTransformers(transformers);
   }
 }