demo.html

<!DOCTYPE html>
<html>
    <link rel="import" href="./attribute-editor.html">
    <link rel="import" href="bower_components/polymer/polymer.html">
    <body>
        <template is="dom-bind" id="Demo">
            <attribute-editor
                usercontent="[[usercontent]]"
                path="[[path]]"
                content-id="[[contentId]]"
                on-value-changed="handleChange"></attribute-editor>
            <button on-click="handleClick">Click me</button>
        </template>

        <script>
            Demo.handleClick = function() {
                document.querySelector('attribute-editor').open();
            }

            Demo.usercontent = {
                'alt': 'styff',
                'src': 'styff'
            }

            Demo.contentId = 'image-url';

            Demo.path = '/some/random/path';

            Demo.handleChange = function(e, detail) {
                console.log(Demo.usercontent);
            }
        </script>
    </body>
</html>