export class Popup and Soud of Iframe Api

let become const in SoundManager

front/src/Phaser/Game/SoundManager.ts

@@ -12,7 +12,7 @@ class SoundManager {
         }
         soundPromise =  new Promise<BaseSound>((res) => {
 
-            let sound = soundManager.get(soundUrl);
+            const sound = soundManager.get(soundUrl);
             if (sound !== null) {
                 return res(sound);
             }

front/src/iframe_api.ts

@@ -62,7 +62,7 @@ interface ButtonDescriptor {
     callback: ButtonClickedCallback,
 }
 
-class Popup {
+export class Popup {
     constructor(private id: number) {
     }
 
@@ -79,7 +79,7 @@ class Popup {
     }
 }
 
-class Sound {
+export class Sound {
     constructor(private url: string) {
         // TODO: send a "loadSound" event
         window.parent.postMessage({