function injectScriptTag(script) {
  // Get some kind of target to put the script in.
  // Only valid documents get body/head, so this is a nice way to ignore bad
  // ones. Try to insert as early as possible.
  var targetElement =
      document.documentElement || document.head || document.body;
  if (targetElement) {
    if (targetElement.firstElementChild) {
      targetElement.insertBefore(script, targetElement.firstElementChild);
    } else {
      targetElement.appendChild(script);
    }
    script.parentNode.removeChild(script);
  }
}
Injects a <script> tag into the document as early as possible.
The hope is that the injected contents run before any of the user code.
@param {!Element} script <script> element.