Add support for transformer clusters to barback.

pkg/barback/lib/src/transformer_group.dart

Index: pkg/barback/lib/src/transformer_group.dart
diff --git a/pkg/barback/lib/src/transformer_group.dart b/pkg/barback/lib/src/transformer_group.dart
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+++ b/pkg/barback/lib/src/transformer_group.dart
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+// 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;
+
+import 'dart:async';
+
+/// 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();
+}