[Flow.js] Typed children

How about add custom children type `children: React$NativeNode` for many react native component:
```js
declare type React$NativeNode =
  | null
  | React$Element<any>
  | Iterable<?React$Node>;
```

It was really useful for the next case:
```js
<View>'test'</View>; // error
<View>{0 && <View id={0}>}</View> // error 
```