message: Ensured replacing scene shows as unmounted
  Navigating from A --> B to A --> C then the fromUnmounted of C should be true
diff: @@ -49,7 +49,7 @@ class NavigationMotion extends React.Component<NavigationMotionProps, Navigation
  var {stateNavigator} = this.props;
  var {keys} = this.state;
  var {crumbs, nextCrumb, oldUrl} = stateNavigator.stateContext;
  - var backward = oldUrl && oldUrl.split('crumb=').length > crumbs.length;
  + var backward = oldUrl && oldUrl.split('crumb=').length > crumbs.length + 1;
  return crumbs.concat(nextCrumb).map(({state, data, url}, index, crumbsAndNext) => {
  var preCrumbs = crumbsAndNext.slice(0, index);
  var {state: nextState, data: nextData} = crumbsAndNext[index + 1] || {state: undefined, data: undefined};
  