[third_party] add polymer 1.2.3

polymer_1.2.3/bower_components/iron-validatable-behavior/test/test-validatable.html

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 <link rel="import" href="../iron-validatable-behavior.html">
 
 <dom-module id="test-validatable">
   <template>
     <content></content>
   </template>
 </dom-module>
 
 <script>
 
   Polymer({
 
     is: 'test-validatable',
 
     behaviors: [
       Polymer.IronValidatableBehavior
     ]
 
   });
 </script>