Add a "message" field to wrapped barback exceptions.

pkg/barback/lib/src/utils.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.utils;
 
 import 'dart:async';
 import 'dart:typed_data';
 
 import 'package:stack_trace/stack_trace.dart';
@@ skipping 307 matching lines @@
 
   // TODO(nweiz): Implement this using a transformer when issues 18588 and 18586
   // are fixed.
   var subscription;
   var controller = new StreamController(onCancel: () => subscription.cancel());
   subscription = broadcast.listen(controller.add,
       onError: controller.addError,
       onDone: controller.close);
   return controller.stream;
 }
+
+/// A regular expression to match the exception prefix that some exceptions'
+/// [Object.toString] values contain.
+final _exceptionPrefix = new RegExp(r'^([A-Z][a-zA-Z]*)?(Exception|Error): ');
+
+/// Get a string description of an exception.
+///
+/// Many exceptions include the exception class name at the beginning of their
+/// [toString], so we remove that if it exists.
+String getErrorMessage(error) =>
+  error.toString().replaceFirst(_exceptionPrefix, '');