Transition barback's infrastructure to an aggregate-based model.

pkg/barback/lib/src/transformer/transformer.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.transformer.transformer;
 
 import 'dart:async';
 
 import '../asset/asset_id.dart';
 import '../utils.dart';
@@ skipping 55 matching lines @@
     // Allow all files if [primaryExtensions] is not overridden.
     if (allowedExtensions == null) return true;
 
     for (var extension in allowedExtensions.split(" ")) {
       if (id.path.endsWith(extension)) return true;
     }
 
     return false;
   }
 
-  /// Run this transformer on on the primary input specified by [transform].
+  /// Run this transformer on the primary input specified by [transform].
   ///
   /// The [transform] is used by the [Transformer] for two purposes (in
   /// addition to accessing the primary input). It can call `getInput()` to
   /// request additional input assets. It also calls `addOutput()` to provide
   /// generated assets back to the system. Either can be called multiple times,
   /// in any order.
   ///
   /// In other words, a Transformer's job is to find all inputs for a
   /// transform, starting at the primary input, then generate all output assets
   /// and yield them back to the transform.
   ///
   /// If this does asynchronous work, it should return a [Future] that completes
   /// once it's finished.
   apply(Transform transform);
 
   String toString() => runtimeType.toString().replaceAll("Transformer", "");
 }