Update SwitchProps to support typescript 2.3 (#16340)

SwitchProps.children is too strict for projects using typescript 2.3.
With typescript 2.3 the children could be made mandatory, but this will
break backwards compatibility with any ts 2.2 user

types/react-router/index.d.ts

@@ -12,6 +12,7 @@
 //                 Tanguy Krotoff <https://github.com/tkrotoff>
 //                 Huy Nguyen <https://github.com/huy-nguyen>
 //                 Jérémy Fauvel <https://github.com/grmiade>
+//                 Daniel Roth <https://github.com/DaIgeb>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.2
 
@@ -84,7 +85,7 @@ export interface StaticRouterProps {
 
 export class StaticRouter extends React.Component<StaticRouterProps, undefined> {}
 export interface SwitchProps {
-  children?: JSX.Element | JSX.Element[];
+  children?: React.ReactNode;
   location?: H.Location;
 }
 export class Switch extends React.Component<SwitchProps, undefined> {}

types/react-router/test/Switch.tsx

@@ -2,12 +2,18 @@ import * as React from 'react';
 import { BrowserRouter, Redirect, Route, Switch } from 'react-router-dom';
 
 const Home = () => <h2>Home</h2>;
+const About = () => <h2>About</h2>;
+const User = () => <h2>User</h2>;
 
 const SwitchTest = () => (
   <BrowserRouter>
     <Switch>
       <Redirect exact from="/" to="/home"/>
       <Route path="/home" component={Home}/>
+      {[
+        <Route path="/user" component={User}/>,
+        <Route path="/about" component={About}/>
+      ]}
     </Switch>
   </BrowserRouter>
 );