Pre-compute the stack chain in Barback's _WrappedException.

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_id.dart';
 import '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;
 
-  _WrappedException(this.error, StackTrace stackTrace)
-      : this.stackTrace = stackTrace == null ? null :
-            new Chain.forTrace(stackTrace);
+  _WrappedException(error, StackTrace stackTrace)
+      : this.error = error,
+        this.stackTrace = _getChain(error, 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${stackTrace.terse}";
-    }
-
+    if (stackTrace != null) result = "$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;
+}
+
 /// Error wrapping an exception thrown by a transform.
 class TransformerException extends _WrappedException {
   /// The transform that threw the exception.
   final TransformInfo transform;
 
   TransformerException(this.transform, error, StackTrace stackTrace)
       : super(error, stackTrace);
 
   String get _message => "Transform $transform threw error";
 }
@@ skipping 28 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";
 }