Support cross-package transforms in barback.

pkg/barback/lib/src/transform.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.transform;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 32 matching lines @@
   /// For example, with a dart2js transform, the primary input would be the
   /// entrypoint Dart file. All of the other Dart files that that imports
   /// would be secondary inputs.
   AssetId get primaryId => _node.primary.id;
 
   /// Gets the asset for the primary input.
   Future<Asset> get primaryInput => getInput(primaryId);
 
   Transform._(this._node, this._outputs);
 
-  /// Gets the asset for for an input [id].
+  /// Gets the asset for an input [id].
   ///
   /// If an input with that ID cannot be found, throws an
   /// [AssetNotFoundException].
   Future<Asset> getInput(AssetId id) => _node.getInput(id);
 
   /// Stores [output] as the output created by this transformation.
   ///
   /// A transformation can output as many assets as it wants.
   void addOutput(Asset output) {
     // TODO(rnystrom): This should immediately throw if an output with that ID
     // has already been created by this transformer.
     _outputs.add(output);
   }
 }