Expose aggregate transformers in barback.

pkg/barback/lib/src/graph/package_graph.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.graph.package_graph;
 
 import 'dart:async';
 import 'dart:collection';
 
 import '../asset/asset_id.dart';
 import '../asset/asset_node.dart';
 import '../asset/asset_set.dart';
 import '../build_result.dart';
 import '../errors.dart';
 import '../log.dart';
 import '../package_provider.dart';
-import '../transformer/transformer.dart';
 import '../utils.dart';
 import 'asset_cascade.dart';
 import 'node_status.dart';
 
 /// The collection of [AssetCascade]s for an entire application.
 ///
 /// This tracks each package's [AssetCascade] and routes asset requests between
 /// them.
 class PackageGraph {
   /// The provider that exposes asset and package information.
@@ skipping 149 matching lines @@
       var cascade = _cascades[package];
       if (cascade == null) throw new ArgumentError("Unknown package $package.");
       _inErrorZone(() => cascade.removeSources(ids));
     });
 
     // It's possible for removing sources not to cause any processing. The user
     // still expects there to be a build, though, so we emit one immediately.
     _tryScheduleResult();
   }
 
-  void updateTransformers(String package,
-      Iterable<Iterable<Transformer>> transformers) {
+  void updateTransformers(String package, Iterable<Iterable> transformers) {
     _inErrorZone(() => _cascades[package].updateTransformers(transformers));
 
     // It's possible for updating transformers not to cause any processing. The
     // user still expects there to be a build, though, so we emit one
     // immediately.
     _tryScheduleResult();
   }
 
   /// A handler for a log entry from an [AssetCascade].
   void _onLog(LogEntry entry) {
@@ skipping 56 matching lines @@
         completer.completeError(error, stackTrace);
       });
     }, onError: (error, stackTrace) {
       _lastUnexpectedError = error;
       _lastUnexpectedErrorTrace = stackTrace;
       _resultsController.addError(error, stackTrace);
     });
     return completer.future;
   }
 }