Make barback more package-aware.

pkg/barback/lib/src/asset_provider.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.asset_provider;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
+import 'transformer.dart';
 
 /// API for locating and accessing packages on disk.
 ///
 /// Implemented by pub and provided to barback so that it isn't coupled
 /// directly to pub.
 abstract class AssetProvider {
   /// The names of all packages that can be provided by this provider.
   ///
   /// This is equal to the transitive closure of the entrypoint package
   /// dependencies.
   Iterable<String> get packages;
 
   // TODO(rnystrom): Make this async.
   /// The paths of all available asset files in [package], relative to the
   /// package's root directory.
   ///
   /// You can pass [within], which should be the relative path to a directory
   /// within the package, to only return the files within that subdirectory.
   List<AssetId> listAssets(String package, {String within});
 
+  /// Returns the list of transformer phases that are applicable to [package].
+  ///
+  /// The phases will be run in sequence, with the outputs of one pipelined into
+  /// the next. All [Transformer]s in a single phase will be run in parallel.
+  Iterable<Iterable<Transformer>> getTransformers(String package);
+
   Future<Asset> getAsset(AssetId id);
 }