Enable flow in react-native-implementation

Summary:
I noticed I didn't get type defs anymore for react-native. Looks like it is broken since we removed the .flow file in 3e153b2a5b. To fix it we can now enable flow in react-native-implementation since it now supports properties.

**Test plan**
Tested that I get type hints when using imports from react-native in a project.
Closes https://github.com/facebook/react-native/pull/12917

Differential Revision: D4704753

Pulled By: ericvicenti

fbshipit-source-id: cf882588d7f371931de8d7861a1a6d50f6c425dc

Libraries/Experimental/SwipeableRow/SwipeableListView.js

@@ -18,6 +18,11 @@ const SwipeableRow = require('SwipeableRow');
 
 const {PropTypes} = React;
 
+type DefaultProps = {
+  bounceFirstRowOnMount: boolean,
+  renderQuickActions: Function,
+};
+
 type Props = {
   bounceFirstRowOnMount: boolean,
   dataSource: SwipeableListViewDataSource,
@@ -49,7 +54,7 @@ type State = {
  * - It can bounce the 1st row of the list so users know it's swipeable
  * - More to come
  */
-class SwipeableListView extends React.Component {
+class SwipeableListView extends React.Component<DefaultProps, Props, State> {
   props: Props;
   state: State;