Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/polymer_expressions/async.dart

Index: observatory_pub_packages/polymer_expressions/async.dart
===================================================================
--- observatory_pub_packages/polymer_expressions/async.dart	(revision 0)
+++ observatory_pub_packages/polymer_expressions/async.dart	(working copy)
@@ -0,0 +1,17 @@
+// 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.async;
+
+import 'dart:async';
+import 'package:observe/observe.dart';
+
+class StreamBinding<T> extends ObservableBox {
+  final Stream<T> stream;
+
+  StreamBinding(this.stream) {
+    stream.listen((T i) { value = i; });
+  }
+
+}