Reorganize barback's source files.

pkg/barback/lib/src/declaring_transformer.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_transformer;
-
-import 'dart:async';
-
-import 'declaring_transform.dart';
-
-/// An interface for [Transformer]s that can cheaply figure out which assets
-/// they'll emit without doing the work of actually creating those assets.
-///
-/// If a transformer implements this interface, that allows barback to perform
-/// optimizations to make the asset graph work more smoothly.
-abstract class DeclaringTransformer {
-  /// Declare which assets would be emitted for the primary input id specified
-  /// by [transform].
-  ///
-  /// This works a little like [Transformer.apply], with two main differences.
-  /// First, instead of having access to the primary input's contents, it only
-  /// has access to its id. Second, instead of emitting [Asset]s, it just emits
-  /// [AssetId]s through [transform.addOutputId].
-  ///
-  /// If this does asynchronous work, it should return a [Future] that completes
-  /// once it's finished.
-  declareOutputs(DeclaringTransform transform);
-}