Ensure that errors have stack traces attached.

sdk/lib/_internal/pub/lib/src/barback.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 pub.barback;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
+import 'package:stack_trace/stack_trace.dart';
 
 import 'barback/load_all_transformers.dart';
 import 'barback/pub_package_provider.dart';
 import 'barback/server.dart';
 import 'barback/sources.dart';
 import 'log.dart' as log;
 import 'package_graph.dart';
 import 'utils.dart';
 
 export 'barback/sources.dart' show WatcherType;
@@ skipping 105 matching lines @@
       loadSources(graph, barback);
     }).then((_) {
       var completer = new Completer();
 
       // If any errors get emitted either by barback or by the server, including
       // non-programmatic barback errors, they should take down the whole
       // program.
       var subscriptions = [
         server.barback.errors.listen((error) {
           if (error is TransformerException) error = error.error;
-          if (!completer.isCompleted) completer.completeError(error);
+          if (!completer.isCompleted) {
+            completer.completeError(error, new Trace.current());
+          }
         }),
         server.barback.results.listen((_) {}, onError: (error, stackTrace) {
           if (completer.isCompleted) return;
           completer.completeError(error, stackTrace);
         }),
         server.results.listen((_) {}, onError: (error, stackTrace) {
           if (completer.isCompleted) return;
           completer.completeError(error, stackTrace);
         })
       ];
@@ skipping 154 matching lines @@
 
     case LogLevel.WARNING:
       log.warning("${log.yellow(prefix)}\n$message");
       break;
 
     case LogLevel.INFO:
       log.message("$prefix\n$message");
       break;
   }
 }