Make Assets know their ID.

pkg/barback/lib/src/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;
 
 import 'dart:async';
 import 'dart:io';
 
-import 'asset_id.dart';
+import 'asset.dart';
 import 'transform.dart';
 
 /// A [Transformer] represents a processor that takes in one or more input
 /// assets and uses them to generate one or more output assets.
 ///
 /// Dart2js, a SASS->CSS processor, a CSS spriter, and a tool to concatenate
 /// files are all examples of transformers. To define your own transformation
 /// step, extend (or implement) this class.
 abstract class Transformer {
   /// Returns `true` if [input] can be a primary input for this transformer.
   ///
   /// While a transformer can read from multiple input files, one must be the
   /// "primary" input. This asset determines whether the transformation should
   /// be run at all. If the primary input is removed, the transformer will no
   /// longer be run.
   ///
   /// A concrete example is dart2js. When you run dart2js, it will traverse
   /// all of the imports in your Dart source files and use the contents of all
   /// of those to generate the final JS. However you still run dart2js "on" a
   /// single file: the entrypoint Dart file that has your `main()` method.
   /// This entrypoint file would be the primary input.
-  Future<bool> isPrimary(AssetId input);
+  Future<bool> isPrimary(Asset input);
 
   /// Run this transformer on 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.
   Future apply(Transform transform);
 
   String toString() => runtimeType.toString().replaceAll("Transformer", "");
 }