fix function type for flow (#25965)

These files fail CI `yarn_flow`.

Author: mondaychen

packages/react-devtools-shell/src/perf-regression/apps/LargeSubtree.js

@@ -15,7 +15,7 @@ function generateArray(size) {
 
 const arr = generateArray(50000);
 
-export default function LargeSubtree() {
+export default function LargeSubtree(): React.Node {
   const [showList, setShowList] = React.useState(false);
   const toggleList = () => {
     const startTime = performance.now();

packages/react-devtools-shell/src/perf-regression/apps/index.js

@@ -10,7 +10,7 @@
 import * as React from 'react';
 import LargeSubtree from './LargeSubtree';
 
-export default function Home() {
+export default function Home(): React.Node {
   return (
     <div>
       <LargeSubtree />