Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/polymer_expressions/filter.dart

Index: observatory_pub_packages/polymer_expressions/filter.dart
===================================================================
--- observatory_pub_packages/polymer_expressions/filter.dart	(revision 0)
+++ observatory_pub_packages/polymer_expressions/filter.dart	(working copy)
@@ -0,0 +1,24 @@
+// 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_expressions.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/*<V, T>*/(this);
+}
+
+// TODO(jmesserly): restore types when Issue 14094 is fixed.
+class _InverseTransformer/*<V, T>*/ implements Transformer/*<V, T>*/ {
+  final Transformer/*<T, V>*/ _t;
+  _InverseTransformer(this._t);
+
+  /*V*/ forward(/*T*/ v) => _t.reverse(v);
+  /*T*/ reverse(/*V*/ t) => _t.forward(t);
+  Transformer/*<T, V>*/ get inverse => _t;
+}