lazyLoading-1.js

var webComponentsSupported = ('registerElement' in document
  && 'import' in document.createElement('link')
  && 'content' in document.createElement('template'));

if (!webComponentsSupported) {
  var wcPoly = document.createElement('script');
  wcPoly.src = '/third_party/webcomponents-lite.min.js';
  wcPoly.onload = lazyLoadPolymerAndElements;
  document.head.appendChild(wcPoly);
} else {
  lazyLoadPolymerAndElements();
}

function lazyLoadPolymerAndElements() {
  var elements = [
    '/path/to/bundle/one.html',
    '/path/to/bundle/two.html'
  ];
  
  elements.forEach(function(elementURL) {
    var elImport = document.createElement('link');
    elImport.rel = 'import';
    elImport.href = elementURL;
    document.head.appendChild(elImport);
  })
}