move fancy_syntax into Dart SVN

pkg/polymer_expression/lib/filter.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 polymer_expression.filter;
+
+typedef Object Filter(Object value);
+
+abstract class Transformer<T, V> {
+
+  T forward(V v);
+  V reverse(T t);
+  Transformer<V, T> get inverse => new _InverseTransformer(this);
+}
+
+class _InverseTransformer<T, V> implements Transformer<T, V> {
+  final Transformer<V, T> _t;
+  _InverseTransformer(this._t);
+
+  T forward(V v) => _t.reverse(v);
+  V reverse(T t) => _t.forward(t);
+  Transformer<V, T> get inverse => _t;
+}