unmap method
Unmap from this instance the pair domNode and element.
Implementation
bool unmap(DOMNode domNode, T element) {
  var prev = _domNodeToElementMap[domNode];
  if (prev == element) {
    _domNodeToElementMap.remove(domNode);
    _elementToDOMNodeMap.remove(prev);
    return true;
  }
  return false;
}