Add chain support to barback.

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 'dart:async';
-
 import 'package:stack_trace/stack_trace.dart';
 
 import 'asset_id.dart';
 import 'transformer.dart';
 import 'utils.dart';
 
 /// Error thrown when an asset with [id] cannot be found.
 class AssetNotFoundException implements Exception {
   final AssetId id;
 
@@ skipping 89 matching lines @@
   InvalidOutputException(this.transform, this.id);
 
   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 StackTrace stackTrace;
+  final Chain stackTrace;
 
-  _WrappedException(this.error, this.stackTrace);
+  _WrappedException(this.error, StackTrace stackTrace)
+      : this.stackTrace = stackTrace == null ? null :
+            new Chain.forTrace(stackTrace);
 
   String get _message;
 
   String toString() {
     var result = "$_message: $error";
 
     var stack = stackTrace;
     if (stack == null && error is Error) stack = error.stackTrace;
     if (stack != null) {
-      result = "$result\n${new Trace.from(stack).terse}";
+      result = "$result\n${stackTrace.terse}";
     }
 
     return result;
   }
 }
 
 /// Error wrapping an exception thrown by a transform.
 class TransformerException extends _WrappedException {
   /// The transform that threw the exception.
   final TransformInfo transform;
@@ skipping 34 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";
 }