message: Prevented re-rendering crumb scenes
  When shared elements came in, it brought in context. Turns out
  ReactReconciler always re-renders when context is there. Before context
  it the reconciler short circuited the update because the scenes where
  === identical. So, cached the url in constructor and added a
  shouldComponentUpdate to avoid re-rendering crumb scenes
diff: @@ -9,8 +9,18 @@ const nextDirection = {
  West: 'North',
  };
  -export default ({direction, color, stateNavigator}) => {
  - const {url, crumbs} = stateNavigator.stateContext;
  +export default class Scene extends React.Component {
  + constructor(props, context) {
  + super(props, context);
  + const {url, crumbs} = props.stateNavigator.stateContext;
  + this.url = url;
  + this.crumbs = crumbs;
  + }
  + shouldComponentUpdate(props) {
  + return this.url === props.stateNavigator.stateContext.url;
  + }
  + render() {
  + const {direction, color, stateNavigator} = this.props;
  return (
  <View style={[
  styles.scene,
  @@ -20,22 +30,23 @@ export default ({direction, color, stateNavigator}) => {
  <TouchableHighlight
  underlayColor={color}
  onPress={() => {
  - if (url === stateNavigator.stateContext.url)
  + if (this.url === stateNavigator.stateContext.url)
  stateNavigator.navigate(`scene${nextDirection[direction]}`);
  }}>
  - <Text style={styles.text}>{direction} {crumbs.length}</Text>
  + <Text style={styles.text}>{direction} {this.crumbs.length}</Text>
  </TouchableHighlight>
  {stateNavigator.canNavigateBack(1) && <TouchableHighlight
  underlayColor={color}
  onPress={() => {
  - if (url === stateNavigator.stateContext.url)
  + if (this.url === stateNavigator.stateContext.url)
  stateNavigator.navigateBack(1);
  }}>
  <Text style={styles.text}>Back</Text>
  </TouchableHighlight>}
  </View>
  )
  -};
  + }
  +}
  const styles = StyleSheet.create({
  scene: {
  