Add support for transformer clusters to barback.

pkg/barback/lib/src/transformer_cluster.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_cluster;
+
+import 'dart:async';
+
+import 'operator.dart';
+
+/// A [TransformerCluster] encapsulates a phased collection of transformers.
+///
+/// A transformer cluster is defined like a collection of phases, such as you
+/// might pass to [Barback.updateOperators]. 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 [TransformerCluster], it
+/// works just like a [Transformer]. It can be included in phases, and will run
+/// in parallel to other transformers or clusters in those phases. Other
+/// transformers and clusters will be unable to see any intermediate assets that
+/// are generated by one phase of the cluster and consumed by another. Phases
+/// following that containing the cluster will be able to see its outputs,

[Bob Nystrom, 2013/10/04 21:47:50]

"Phases following that" -> "Phases after the one".

[nweiz, 2013/10/07 23:21:31]

Done.

+/// though.
+class TransformerCluster implements Operator {

[Bob Nystrom, 2013/10/04 21:47:50]

As per our discussion: "TransformerGroup".

[nweiz, 2013/10/07 23:21:31]

Done.

+  /// The phases that comprise this cluster.
+  final Iterable<Iterable<Operator>> phases;
+
+  TransformerCluster(Iterable<Iterable<Operator>> phases)
+      : this.phases = phases.map((phase) => phase.toList()).toList();
+}