foo.js

var onload = function() {
    // If web components are supported this will fire immediately,
    // othewrise it will wait for the polyfills to load.
    // Load your components here
    var components = document.createElement('script');
    script.async = true;
    script.src = '../dist/page-sections.js';
    document.head.appendChild(components);
  };

  var webComponentsSupported = (
    'registerElement' in document
  );

  if (!webComponentsSupported) {
    var script = document.createElement('script');
    script.async = true;
    script.src = '/bower_components/webcomponentsjs/webcomponents-lite.min.js';
    script.onload = onload;
    document.head.appendChild(script);
  } else {
    onload();
  }
})();