Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/browser/interop.js

Index: observatory_pub_packages/browser/interop.js
===================================================================
--- observatory_pub_packages/browser/interop.js	(revision 0)
+++ observatory_pub_packages/browser/interop.js	(working copy)
@@ -0,0 +1,9 @@
+// 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.
+
+// TODO(jmesserly): remove this script after a deprecation period.
+if (typeof console == "object" && typeof console.warn == "function") {
+  console.warn('<script src="packages/browser/interop.js"> is no longer ' +
+      'needed for dart:js. See http://pub.dartlang.org/packages/browser.');
+}