Resolve react flow definitions

Summary:Currently, we're not taking advantage of Flow's built-in type definitions for the React library in all cases because Flow's definition uses `declare module react` and this file uses `import('React')`, which Flow thinks is a different library. After this change, the following starts working which didn't before:

```js
import { Component } from 'react-native';

class MyText extends Component<void, {text: string}, void> {
  render() { return <Text>{this.props.text}</Text> }
}

// Correctly throws a Flow error for the missing "text" prop
const renderedText = <MyText />;
```
Closes https://github.com/facebook/react-native/pull/5489

Differential Revision: D2856176

fb-gh-sync-id: 473ca188ad7d990c3e765526c4b33caf49ad9ffd
shipit-source-id: 473ca188ad7d990c3e765526c4b33caf49ad9ffd

Libraries/react-native/react-native.js.flow

@@ -23,7 +23,7 @@
 //
 //   var ReactNative = {...require('React'), /* additions */}
 //
-var ReactNative = Object.assign(Object.create(require('React')), {
+var ReactNative = Object.assign(Object.create(require('react')), {
   // Components
   ActivityIndicatorIOS: require('ActivityIndicatorIOS'),
   ART: require('ReactNativeART'),