First stab at a dev server in pub using barback.

pkg/barback/lib/src/asset_cascade.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.asset_cascade;
 
 import 'dart:async';
 import 'dart:collection';
 
+import 'package:stack_trace/stack_trace.dart';
+
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_set.dart';
 import 'errors.dart';
 import 'change_batch.dart';
 import 'package_graph.dart';
 import 'phase.dart';
 import 'transformer.dart';
 import 'utils.dart';
 
@@ skipping 267 matching lines @@
       msg.write(prefixLines(error.toString()));
       if (stackTrace != null) {
         msg.write("\n\n");
         msg.write("Stack trace:\n");
         msg.write(prefixLines(stackTrace.toString()));
       }
       return msg.toString();
     }).join("\n\n");
   }
 }