Make barback more package-aware.

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 'dart:io';
 
 import 'asset_id.dart';
@@ skipping 18 matching lines @@
 
 /// Error thrown when a transformer requests an input [id] which cannot be
 /// found.
 class MissingInputException implements Exception {
   final AssetId id;
 
   MissingInputException(this.id);
 
   String toString() => "Missing input $id.";
 }
+
+/// Error thrown when a transformer outputs an asset with the wrong package
+/// name.
+class InvalidOutputException implements Exception {
+  final String package;
+  final AssetId id;
+
+  InvalidOutputException(this.package, this.id);
+
+  String toString() => "Invalid output $id: must be in package $package.";
+}