function fastInnerText(element, content) {
  var child = element.firstChild;

  if (child && child.nodeType === 3 && child.nextSibling === null) {
    // fast lane - replace existing text node
    if (_feature.isTextContentSupported) {
      // http://jsperf.com/replace-text-vs-reuse
      child.textContent = content;
    } else {
      // http://jsperf.com/replace-text-vs-reuse
      child.data = content;
    }
  } else {
    // slow lane - empty element and insert a text node
    empty(element);
    element.appendChild(element.ownerDocument.createTextNode(content));
  }
}
Insert text content into element
@return {Boolean}