function getReferenceNode(referenceId) {
  return dom.getFirstElementChild(
      window.frames['reference'].document.getElementById(referenceId));
}
Returns a reference to the first element child of a node with the given id
from the page loaded into the reference iFrame. Used to retrieve a particular
reference DOM structure to test against.
@param {string} referenceId The id of a container element for a reference
    structure in the reference page.
@return {?Node} The root element of the reference structure.