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/Image/Image.android.js

@@ -194,7 +194,7 @@ var Image = React.createClass({
     getSize(
       url: string,
       success: (width: number, height: number) => void,
-      failure: (error: any) => void,
+      failure?: (error: any) => void,
     ) {
       return ImageLoader.getSize(url)
         .then(function(sizes) {

Libraries/Image/Image.ios.js

@@ -304,7 +304,7 @@ const Image = React.createClass({
     getSize: function(
       uri: string,
       success: (width: number, height: number) => void,
-      failure: (error: any) => void,
+      failure?: (error: any) => void,
     ) {
       ImageViewManager.getSize(uri, success, failure || function() {
         console.warn('Failed to get size for image: ' + uri);