Add license info to pkgs so we can pub lish directly

pkg/polymer_expressions/PATENTS

+Additional IP Rights Grant (Patents)
+
+"This implementation" means the copyrightable works distributed by
+Google as part of the Dart Project.
+
+Google hereby grants to you a perpetual, worldwide, non-exclusive,
+no-charge, royalty-free, irrevocable (except as stated in this
+section) patent license to make, have made, use, offer to sell, sell,
+import, transfer, and otherwise run, modify and propagate the contents
+of this implementation of Dart, where such license applies only to
+those patent claims, both currently owned by Google and acquired in
+the future, licensable by Google that are necessarily infringed by
+this implementation of Dart. This grant does not include claims that
+would be infringed only as a consequence of further modification of
+this implementation. If you or your agent or exclusive licensee
+institute or order or agree to the institution of patent litigation
+against any entity (including a cross-claim or counterclaim in a
+lawsuit) alleging that this implementation of Dart or any code
+incorporated within this implementation of Dart constitutes direct or
+contributory patent infringement, or inducement of patent
+infringement, then any patent rights granted to you under this License
+for this implementation of Dart shall terminate as of the date such
+litigation is filed.