Improve flow typing

Summary:
- Properly inherit flow types from base components, including `defaultProps`
- template-ify the `Item` type as it flows from the `data` prop into `ItemComponent`

Note that for `SectionList` is is harder to do the `Item` typing because each section in the
`sections` array can have a different `Item` type, plus all the optional overrides...not sure how to
tackle that.

Reviewed By: yungsters

Differential Revision: D4557523

fbshipit-source-id: a0c5279fcdaabe6aab5fe11743a99c3715a44032

Examples/UIExplorer/js/FlatListExample.js

@@ -108,7 +108,7 @@ class FlatListExample extends React.PureComponent {
           key={(this.state.horizontal ? 'h' : 'v') + (this.state.fixedHeight ? 'f' : 'd')}
           legacyImplementation={false}
           numColumns={1}
-          onRefresh={() => alert('onRefresh: nothing to refresh :P')}
+          onRefresh={this._onRefresh}
           onViewableItemsChanged={this._onViewableItemsChanged}
           ref={this._captureRef}
           refreshing={false}
@@ -121,6 +121,7 @@ class FlatListExample extends React.PureComponent {
   _getItemLayout = (data: any, index: number) => {
     return getItemLayout(data, index, this.state.horizontal);
   };
+  _onRefresh = () => alert('onRefresh: nothing to refresh :P');
   _renderItemComponent = ({item}) => {
     return (
       <ItemComponent
@@ -154,7 +155,7 @@ class FlatListExample extends React.PureComponent {
   _pressItem = (key: number) => {
     pressItem(this, key);
   };
-  _listRef: FlatList;
+  _listRef: FlatList<*>;
 }