Reorganize barback's source files.

pkg/barback/lib/src/transformer_group.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.transformer_group;
-
-/// A [TransformerGroup] encapsulates a phased collection of transformers.
-///
-/// A transformer group is defined like a collection of phases, such as you
-/// might pass to [Barback.updateTransformers]. Input assets are transformed by
-/// the first phase, whose results are passed to the second phase, and so on.
-///
-/// However, from the perspective of someone using a [TransformerGroup], it
-/// works just like a [Transformer]. It can be included in phases, and will run
-/// in parallel to other transformers or groups in those phases. Other
-/// transformers and groups will be unable to see any intermediate assets that
-/// are generated by one phase of the group and consumed by another. Phases
-/// after the one containing the group will be able to see its outputs, though.
-class TransformerGroup {
-  /// The phases that comprise this group.
-  ///
-  /// Each element of the inner iterable must be either a [Transformer] or a
-  /// [TransformerGroup].
-  final Iterable<Iterable> phases;
-
-  TransformerGroup(Iterable<Iterable> phases)
-      : this.phases = phases.map((phase) => phase.toList()).toList();
-
-  String toString() => "group of $phases";
-}