Add a "message" field to wrapped barback exceptions.

pkg/barback/lib/src/errors.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.errors;
 
 import 'package:stack_trace/stack_trace.dart';
 
 import 'asset/asset_id.dart';
 import 'transformer/wrapping_aggregate_transformer.dart';
@@ skipping 97 matching lines @@
   String toString() => "Transform $transform emitted $id, which wasn't in the "
       "same package (${transform.primaryId.package}).";
 }
 
 /// Base class for an error that wraps another.
 abstract class _WrappedException implements BarbackException {
   /// The wrapped exception.
   final error;
   final Chain stackTrace;
 
+  String get message => "$_message: ${getErrorMessage(error)}";
+
+  String get _message;
+
   _WrappedException(error, StackTrace stackTrace)
       : this.error = error,
         this.stackTrace = _getChain(error, stackTrace);
 
-  String get _message;
-
   String toString() {
-    var result = "$_message: $error";
+    var result = message;
     if (stackTrace != null) result = "$result\n${stackTrace.terse}";
     return result;
   }
 }
 
 /// Returns the stack chain for [error] and [stackTrace].
 Chain _getChain(error, StackTrace stackTrace) {
   if (error is Error && stackTrace == null) stackTrace = error.stackTrace;
   if (stackTrace != null) return new Chain.forTrace(stackTrace);
   return null;
@@ skipping 45 matching lines @@
 
   bool operator==(other) =>
       other is TransformInfo &&
       other.transformer == transformer &&
       other.primaryId == primaryId;
 
   int get hashCode => transformer.hashCode ^ primaryId.hashCode;
 
   String toString() => "$transformer on $primaryId";
 }