fix TypeError on iOS(cordova) with polymer.dart 0.12.0 that include polymer.js 0.3.4-370b65f.

[Error] TypeError: 'undefined' is not an object (evaluating 'b.element') (polymer.js, line 12)

TypeError: 'undefined' is not an object (evaluating 'b.element') in declaration/prototype.js
https://github.com/Polymer/polymer-dev/issues/76