custom.html

<dom-module id="x-foo">
  <div>test</div>
</dom-module>

<script>
  Polymer.X_CONSTANT = 1;
  
  Polymer({
    is: 'x-foo',
    properties: {
      foo: {
        type: String
      }
    },
    
    observers: [
      '_setup(foo)'
    ],
    
    _setup: function() {
      var ctx = Polymer.X_CONSTANT;
      console.debug('ctx', ctx);
      //Polymer.X.CONSTANT = 2;
    }
  });
</script>