Fix ScrollView context warning in Modal.js

Summary: This fixes the following warning from appearing when you have a FlatList render a Modal where the content of the Modal also contains a FlatList: https://fburl.com/p953k985.  Spencer addressed an issue similar to this in D7863625, but we still get a yellow box due to the fact that `scrollContext` still exists, but `this.context.virtualizedList` is null from this line in Modal.js https://fburl.com/nqc261a1.

Reviewed By: cpojer

Differential Revision: D16160666

fbshipit-source-id: ba222d3eef234f4c8c4c2bddbc71bec27df81e0a

Libraries/Modal/Modal.js

@@ -17,6 +17,7 @@ import NativeModalManager from './NativeModalManager';
 const Platform = require('../Utilities/Platform');
 const React = require('react');
 const PropTypes = require('prop-types');
+const ScrollView = require('../Components/ScrollView/ScrollView');
 const StyleSheet = require('../StyleSheet/StyleSheet');
 const View = require('../Components/View/View');
 
@@ -258,7 +259,11 @@ class Modal extends React.Component<Props> {
         onStartShouldSetResponder={this._shouldSetResponder}
         supportedOrientations={this.props.supportedOrientations}
         onOrientationChange={this.props.onOrientationChange}>
-        <View style={[styles.container, containerStyles]}>{innerChildren}</View>
+        <ScrollView.Context.Provider value={null}>
+          <View style={[styles.container, containerStyles]}>
+            {innerChildren}
+          </View>
+        </ScrollView.Context.Provider>
       </RCTModalHostView>
     );
   }