Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/polymer/src/build/constants.dart

Index: observatory_pub_packages/polymer/src/build/constants.dart
===================================================================
--- observatory_pub_packages/polymer/src/build/constants.dart	(revision 0)
+++ observatory_pub_packages/polymer/src/build/constants.dart	(working copy)
@@ -0,0 +1,20 @@
+// Copyright (c) 2014, 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.src.build.constants;
+
+/// These names have meaning in SVG or MathML, so they aren't allowed as custom
+/// tags. See [isCustomTagName].
+const invalidTagNames = const {
+  'annotation-xml': '',
+  'color-profile': '',
+  'font-face': '',
+  'font-face-src': '',
+  'font-face-uri': '',
+  'font-face-format': '',
+  'font-face-name': '',
+  'missing-glyph': '',
+};
+
+const POLYMER_EXPERIMENTAL_HTML = 'packages/polymer/polymer_experimental.html';