code: function importElements(node, callback) {
    if (node) {
      document.head.appendChild(node);
      whenReady(callback);
    } else if (callback) {
      callback();
    }
  }
docs: Loads the set of HTMLImports contained in `node`. Notifies when all
  the imports have loaded by calling the `callback` function argument.
  This method can be used to lazily load imports. For example, given a
  template:
  
  <template>
  <link rel="import" href="my-import1.html">
  <link rel="import" href="my-import2.html">
  </template>
  
  Polymer.importElements(template.content, function() {
  console.log('imports lazily loaded');
  });
  
  @method importElements
  @param {Node} node Node containing the HTMLImports to load.
  @param {Function} callback Callback called when all imports have loaded.