Add support for lazy transformers.

pkg/barback/lib/src/base_transform.dart

Index: pkg/barback/lib/src/base_transform.dart
diff --git a/pkg/barback/lib/src/transform.dart b/pkg/barback/lib/src/base_transform.dart
similarity index 62%
copy from pkg/barback/lib/src/transform.dart
copy to pkg/barback/lib/src/base_transform.dart
index 7f0d217b072f1fd34f03ca87fc5d6b08f86af3ea..27dc12a3a1c1bf33a0c388eff562956705e98752 100644
--- a/pkg/barback/lib/src/transform.dart
+++ b/pkg/barback/lib/src/base_transform.dart
@@ -1,47 +1,28 @@
-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2014, 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;
+library barback.base_transform;
 
 import 'dart:async';
 import 'dart:convert';
 
-import 'package:source_maps/span.dart';
-
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
-import 'asset_set.dart';
 import 'errors.dart';
-import 'log.dart';
 import 'transform_logger.dart';
 import 'transform_node.dart';
 import 'utils.dart';
 
-typedef void LogFunction(AssetId asset, LogLevel level, String message,
-                         Span span);
-
-/// 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,
-                          LogFunction logFunction) =>
-    new Transform._(node, outputs, logFunction);
-
-/// While a [Transformer] represents a *kind* of transformation, this defines
-/// one specific usage of it on a set of files.
+/// The base class for the ephemeral transform objects that are passed to
+/// transformers.
 ///
-/// This ephemeral object exists only during an actual transform application to
-/// facilitate communication between the [Transformer] and the code hosting
-/// the transformation. It lets the [Transformer] access inputs and generate
-/// outputs.
-class Transform {
+/// This class provides the transformers with inputs, but its up to the
+/// subclasses to provide a means of emitting outputs.
+abstract class BaseTransform {
   final TransformNode _node;
   final TransformLogger _logger;
-  final AssetSet _outputs;
 
   /// A logger so that the [Transformer] can report build details.
   TransformLogger get logger => _logger;
@@ -69,7 +50,7 @@ class Transform {
     return _node.primary.asset;
   }
 
-  Transform._(this._node, this._outputs, LogFunction logFunction)
+  BaseTransform(this._node, LogFunction logFunction)
     : _logger = new TransformLogger(logFunction);
 
   /// Gets the asset for an input [id].
@@ -97,13 +78,4 @@ class Transform {
   /// If the asset was created from a [String], this returns its UTF-8 encoding.
   Stream<List<int>> readInput(AssetId id) =>
       futureStream(getInput(id).then((input) => input.read()));
-
-  /// 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);
-  }
 }