Only pass an AssetId to isPrimary and declareOutputs.

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 'asset.dart';
+import 'asset_id.dart';
 import 'transform.dart';
 import 'utils.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.
 ///
@@ skipping 16 matching lines @@
     var invalidExtensions = allowedExtensions.split(" ")
         .where((extension) => !extension.startsWith("."))
         .map((extension) => '"$extension"');
     if (invalidExtensions.isEmpty) return;
 
     throw new FormatException('Each extension in $this.allowedExtensions '
         'must begin with a ".", but ${toSentence(invalidExtensions)} '
         '${pluralize("doesn't", invalidExtensions.length, plural: "don't")}.');
   }
 
-  /// Returns `true` if [input] can be a primary input for this transformer.
+  /// Returns `true` if [inputId] 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.
   ///
   /// If this is not overridden, defaults to allow any asset whose extension
   /// matches one of the ones returned by [allowedExtensions]. If *that* is
   /// not overridden, allows all assets.
-  Future<bool> isPrimary(Asset input) {
+  Future<bool> isPrimary(AssetId id) {

[blois, 2014/04/03 21:16:36]

Make it not return a future pretty please!! (or at least make it be a union type
Future<bool>|bool (hah!))

[nweiz, 2014/04/03 21:34:18]

I think we're planning to do the union type thing, but not in this CL.

     // Allow all files if [primaryExtensions] is not overridden.
     if (allowedExtensions == null) return new Future.value(true);
 
     for (var extension in allowedExtensions.split(" ")) {
-      if (input.id.path.endsWith(extension)) return new Future.value(true);
+      if (id.path.endsWith(extension)) return new Future.value(true);
     }
 
     return new Future.value(false);
   }
 
   /// 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", "");
 }