Make observable transform a barback transform.

pkg/barback/lib/src/transform.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.transform;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
 import 'asset_set.dart';
 import 'errors.dart';
+import 'transform_logger.dart';
 import 'transform_node.dart';
 import 'utils.dart';
 
 /// Creates a [Transform] by forwarding to the private constructor.
 ///
 /// Lets [TransformNode] create [Transforms] without giving a [Transform]
 /// itself a public constructor, which would be visible to external users.
 /// Unlike the [Transform] class, this function is not exported by barback.dart.
 Transform createTransform(TransformNode node, AssetSet outputs) =>
     new Transform._(node, outputs);
@@ skipping 14 matching lines @@
   ///
   /// While a transformation can use multiple input assets, one must be a
   /// special "primary" asset. This will be the "entrypoint" or "main" input
   /// file for a transformation.
   ///
   /// For example, with a dart2js transform, the primary input would be the
   /// entrypoint Dart file. All of the other Dart files that that imports
   /// would be secondary inputs.
   AssetId get primaryId => _node.primary.id;
 
+  /// A logger so that the [Transformer] can report build details.
+  TransformLogger get logger => _logger;
+
   /// Gets the asset for the primary input.
   Future<Asset> get primaryInput => getInput(primaryId);
 
   Transform._(this._node, this._outputs);
 
   /// Gets the asset for for an input [id].
   ///
   /// If an input with that ID cannot be found, throws an
   /// [AssetNotFoundException].
   Future<Asset> getInput(AssetId id) => _node.getInput(id);
 
   /// Stores [output] as the output created by this transformation.
   ///
   /// A transformation can output as many assets as it wants.
   void addOutput(Asset output) {
     // TODO(rnystrom): This should immediately throw if an output with that ID
     // has already been created by this transformer.
     _outputs.add(output);
   }
 }
+
+// TODO(sigmund,rnystrom): create a separate logger for each Transfom.
+final TransformLogger _logger = new TransformLogger(true);