Reorganize barback's source files.

pkg/barback/lib/src/declaring_transform.dart

-// 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.declaring_transform;
-
-import 'asset_id.dart';
-import 'base_transform.dart';
-import 'transform_node.dart';
-
-/// A transform for [DeclaringTransform]ers that allows them to declare the ids
-/// of the outputs they'll generate without generating the concrete bodies of
-/// those outputs.
-class DeclaringTransform extends BaseTransform {
-  final _outputIds = new Set<AssetId>();
-
-  final AssetId primaryId;
-
-  DeclaringTransform._(TransformNode node)
-      : primaryId = node.primary.id,
-        super(node);
-
-  /// Stores [id] as the id of an output that will be created by this
-  /// transformation when it's run.
-  ///
-  /// A transformation can declare as many assets as it wants. If
-  /// [DeclaringTransformer.declareOutputs] declareds a given asset id for a
-  /// given input, [Transformer.apply] should emit the corresponding asset as
-  /// well.
-  void declareOutput(AssetId id) {
-    // TODO(nweiz): This should immediately throw if an output with that ID
-    // has already been declared by this transformer.
-    _outputIds.add(id);
-  }
-}
-
-/// The controller for [DeclaringTransform].
-class DeclaringTransformController extends BaseTransformController {
-  DeclaringTransform get transform => super.transform;
-
-  /// The set of ids that the transformer declares it will emit for the given
-  /// primary input.
-  Set<AssetId> get outputIds => transform._outputIds;
-
-  DeclaringTransformController(TransformNode node)
-      : super(new DeclaringTransform._(node));
-}