update polymer version to 0.13

The only changes are to the polymer and core_elements version #s.
Staged at:

https://p13-dot-dart-lang.appspot.com/polymer/
https://p13-dot-dart-lang.appspot.com/docs/tutorials/polymer-intro/

R=sigmund@google.com

Review URL: https://chromiumcodereview.appspot.com/516133003