Add missing class annotations xplat/js

Reviewed By: SamChou19815

Differential Revision: D38373443

fbshipit-source-id: 1222c4845ebd6b72bd6f54af1a27cf8542dd883a

packages/rn-tester/js/examples/TurboModule/SampleTurboModuleExample.js

@@ -129,7 +129,7 @@ class SampleTurboModuleExample extends React.Component<{||}, State> {
       | 'promise'
       | 'rejectPromise'
       | 'voidFunc',
-  ) {
+  ): React.Node {
     const result = this.state.testResults[name] || {};
     return (
       <View style={styles.result}>
@@ -139,7 +139,7 @@ class SampleTurboModuleExample extends React.Component<{||}, State> {
     );
   }
 
-  componentDidMount() {
+  componentDidMount(): void {
     if (global.__turboModuleProxy == null) {
       throw new Error(
         'Cannot load this example because TurboModule is not configured.',