[web] remove unnecessary awaits from flutter.js (#130204)

Fixed types for `_getNewServiceWorker` and `_waitForServiceWorkerActivation` functions.  
These functions currently expect a Promise as an argument, but we're actually passing in an already resolved value:
```js
.then(this._getNewServiceWorker)
.then(this._waitForServiceWorkerActivation);
```

packages/flutter_tools/lib/src/web/file_generators/js/flutter.js

@@ -156,53 +156,46 @@ _flutter.loader = null;
     }
 
     /**
-     * Returns the latest service worker for the given `serviceWorkerRegistrationPromise`.
+     * Returns the latest service worker for the given `serviceWorkerRegistration`.
      *
      * This might return the current service worker, if there's no new service worker
      * awaiting to be installed/updated.
      *
-     * @param {Promise<ServiceWorkerRegistration>} serviceWorkerRegistrationPromise
+     * @param {ServiceWorkerRegistration} serviceWorkerRegistration
      * @returns {Promise<ServiceWorker>}
      */
-    async _getNewServiceWorker(serviceWorkerRegistrationPromise) {
-      const reg = await serviceWorkerRegistrationPromise;
-
-      if (!reg.active && (reg.installing || reg.waiting)) {
+    async _getNewServiceWorker(serviceWorkerRegistration) {
+      if (!serviceWorkerRegistration.active && (serviceWorkerRegistration.installing || serviceWorkerRegistration.waiting)) {
         // No active web worker and we have installed or are installing
         // one for the first time. Simply wait for it to activate.
         console.debug("Installing/Activating first service worker.");
-        return reg.installing || reg.waiting;
-      } else if (!reg.active.scriptURL.endsWith(serviceWorkerVersion)) {
+        return serviceWorkerRegistration.installing || serviceWorkerRegistration.waiting;
+      } else if (!serviceWorkerRegistration.active.scriptURL.endsWith(serviceWorkerVersion)) {
         // When the app updates the serviceWorkerVersion changes, so we
         // need to ask the service worker to update.
-        return reg.update().then((newReg) => {
-          console.debug("Updating service worker.");
-          return newReg.installing || newReg.waiting || newReg.active;
-        });
+        const newRegistration = await serviceWorkerRegistration.update();
+        console.debug("Updating service worker.");
+        return newRegistration.installing || newRegistration.waiting || newRegistration.active;
       } else {
         console.debug("Loading from existing service worker.");
-        return reg.active;
+        return serviceWorkerRegistration.active;
       }
     }
 
     /**
-     * Returns a Promise that resolves when the `latestServiceWorker` changes its
+     * Returns a Promise that resolves when the `serviceWorker` changes its
      * state to "activated".
      *
-     * @param {Promise<ServiceWorker>} latestServiceWorkerPromise
+     * @param {ServiceWorker} serviceWorker
      * @returns {Promise<void>}
      */
-    async _waitForServiceWorkerActivation(latestServiceWorkerPromise) {
-      const serviceWorker = await latestServiceWorkerPromise;
-
+    async _waitForServiceWorkerActivation(serviceWorker) {
       if (!serviceWorker || serviceWorker.state == "activated") {
         if (!serviceWorker) {
-          return Promise.reject(
-            new Error("Cannot activate a null service worker!")
-          );
+          throw new Error("Cannot activate a null service worker!");
         } else {
           console.debug("Service worker already active.");
-          return Promise.resolve();
+          return;
         }
       }
       return new Promise((resolve, _) => {