Add support for transformer clusters to barback.

pkg/barback/lib/src/operator.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.operator;
+
+import 'transformer.dart';
+
+/// An interface that denotes classes that can be used as Barback operators.

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

As per our discussion, this doesn't add much value. Let's just ditch it and make
the API that takes Transformer | Cluster just be untyped. Everywhere you use
"operator" just switch it back to "transformer".

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

Done.

+///
+/// By far the most common type of operator is the [Transformer]. If you aren't
+/// sure what type of operator you want, that's the one.
+///
+/// External libraries shouldn't implement or extend Operator directly.
+abstract class Operator {
+}