[react] Add types for experimental_useEvent

- [`experimental_useEvent` entrypoint](https://unpkg.com/browse/[email protected]/cjs/react.development.js)
- [implementation](https://unpkg.com/browse/[email protected]/cjs/react-dom.development.js)
- [experimental_useEvent](facebook/react#25229)

Author: std4453

types/react/experimental.d.ts

@@ -129,4 +129,7 @@ declare module '.' {
     export type Usable<T> = Thenable<T> | Context<T>;
 
     export function experimental_use<T>(usable: Usable<T>): T;
+
+    // tslint:disable-next-line ban-types
+    export function experimental_useEvent<T extends Function>(event: T): T;
 }

types/react/test/hooks.tsx

@@ -263,6 +263,32 @@ const everyHookRef = React.createRef<{ id: number }>();
  }}/>;
 
 function useExperimentalHooks() {
+    // Implicit any
+    const anyEvent = React.experimental_useEvent(value => {
+        // $ExpectType any
+        return value;
+    });
+    // $ExpectType any
+    anyEvent({});
+    // $ExpectType (value: string) => number
+    const typedEvent = React.experimental_useEvent((value: string) => {
+        return Number(value);
+    });
+    // $ExpectType number
+    typedEvent('1');
+    // Argument of type '{}' is not assignable to parameter of type 'string'.
+    // @ts-expect-error
+    typedEvent({});
+
+    function useContextuallyTypedEvent(fn: (event: Event) => string) {}
+    useContextuallyTypedEvent(
+        React.experimental_useEvent(event => {
+            // $ExpectType Event
+            event;
+            return String(event);
+        }),
+    );
+
     const [toggle, setToggle] = React.useState(false);
 
     const [done, startTransition] = React.useTransition();