function startDOMObserver(editor: DraftEditor) {
  if (!domObserver) {
    domObserver = new DOMObserver(getContentEditableContainer(editor));
    domObserver.start();
  }
}
A handful of variables used to track the current composition and its
resolution status. These exist at the module level because it is not
possible to have compositions occurring in multiple editors simultaneously,
and it simplifies state management with respect to the DraftEditor component.