feat(react): add StrictMode definition (#24629)

types/react/index.d.ts

@@ -1,4 +1,4 @@
-// Type definitions for React 16.1
+// Type definitions for React 16.3
 // Project: http://facebook.github.io/react/
 // Definitions by: Asana <https://asana.com>
 //                 AssureSign <http://www.assuresign.com>
@@ -16,6 +16,7 @@
 //                 Josh Rutherford <https://github.com/theruther4d>
 //                 Guilherme Hübner <https://github.com/guilhermehubner>
 //                 Josh Goldberg <https://github.com/joshuakgoldberg>
+//                 Johann Rakotoharisoa <https://github.com/jrakotoharisoa>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.6
 
@@ -268,6 +269,7 @@ declare namespace React {
 
     const Children: ReactChildren;
     const Fragment: ComponentType;
+    const StrictMode: ComponentType;
     const version: string;
 
     //

types/react/test/tsx.tsx

@@ -89,6 +89,13 @@ const StatelessComponentWithoutProps: React.SFC = (props) => {
     </React.Fragment>
 </div>;
 
+// Strict Mode
+<div>
+    <React.StrictMode>
+        <div />
+    </React.StrictMode>
+</div>;
+
 // Below tests that setState() works properly for both regular and callback modes
 class SetStateTest extends React.Component<{}, { foo: boolean, bar: boolean }> {
     handleSomething = () => {