code: function _import(urls, callback) {
    if (urls && urls.length) {
        var frag = document.createDocumentFragment();
        for (var i=0, l=urls.length, url, link; (i<l) && (url=urls[i]); i++) {
          link = document.createElement('link');
          link.rel = 'import';
          link.href = url;
          frag.appendChild(link);
        }
        importElements(frag, callback);
    } else if (callback) {
      callback();
    }
  }
docs: Loads an HTMLImport for each url specified in the `urls` array.
  Notifies when all the imports have loaded by calling the `callback`
  function argument. This method can be used to lazily load imports.
  For example,
  
  Polymer.import(['my-import1.html', 'my-import2.html'], function() {
  console.log('imports lazily loaded');
  });
  
  @method import
  @param {Array} urls Array of urls to load as HTMLImports.
  @param {Function} callback Callback called when all imports have loaded.