x-test.html

 <dom-module id="x-test">
      <template>
        <div>[[test]]</div>
      </template>
      <script>
          Polymer({
            is: 'x-test',
            properties: {
              test: {
                type: String,
                value: 'foo'
              },
              
              changeTest: {
               type: Boolean
              }
            },
            observers: ['_observer(changeTest)'],
            _observer: function() {
              this.test = 'bar';
            }
          });
      </script>
    </dom-module>