Direct assignment of the ref to avoid warning when mocking.

Summary:
When Jest is mocking native components that are assigned a ref with an arrow function, a warning is generated about stateless components which can not have a ref. This does not happen when the ref is assigned directly.

Fixes #16045.
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The test provided in the related issue is used to validate the fix. I don't know how to detect console warnings in a Jest test.
Closes https://github.com/facebook/react-native/pull/16046

Differential Revision: D6017903

Pulled By: ericnakagawa

fbshipit-source-id: a7ed61c39f141a4094f08fc875289a7a79ebe9e8

jest/setup.js

@@ -313,9 +313,13 @@ jest
 jest.doMock('requireNativeComponent', () => {
   const React = require('react');
 
-  return viewName => props => React.createElement(
-    viewName,
-    props,
-    props.children,
-  );
+  return viewName => class extends React.Component {
+    render() {
+      return React.createElement(
+        viewName,
+        this.props,
+        this.props.children,
+      );
+    }
+  };
 });