Fix unhandled rejection on route change in dev (vercel#31554)

This ensures we properly catch the rejection on route change failure in development so the dev overlay doesn't flash with the unhandled rejection. A test case isn't able to be added for this specific scenario as the unhandled rejection is fired right before the page is navigated so we can't consistently check if the rejection was unhandled or not. 

## Bug

- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

Fixes: vercel#28560

packages/next/client/route-loader.ts

@@ -376,34 +376,30 @@ export function createRouteLoader(assetPrefix: string): RouteLoader {
     },
     loadRoute(route: string, prefetch?: boolean) {
       return withFuture<RouteLoaderEntry>(route, routes, () => {
-        const routeFilesPromise = getFilesForRoute(assetPrefix, route)
-          .then(({ scripts, css }) => {
-            return Promise.all([
-              entrypoints.has(route)
-                ? []
-                : Promise.all(scripts.map(maybeExecuteScript)),
-              Promise.all(css.map(fetchStyleSheet)),
-            ] as const)
-          })
-          .then((res) => {
-            return this.whenEntrypoint(route).then((entrypoint) => ({
-              entrypoint,
-              styles: res[1],
-            }))
-          })
+        let devBuildPromiseResolve: () => void
 
         if (process.env.NODE_ENV === 'development') {
           devBuildPromise = new Promise<void>((resolve) => {
-            if (routeFilesPromise) {
-              return routeFilesPromise.finally(() => {
-                resolve()
-              })
-            }
+            devBuildPromiseResolve = resolve
           })
         }
 
         return resolvePromiseWithTimeout(
-          routeFilesPromise,
+          getFilesForRoute(assetPrefix, route)
+            .then(({ scripts, css }) => {
+              return Promise.all([
+                entrypoints.has(route)
+                  ? []
+                  : Promise.all(scripts.map(maybeExecuteScript)),
+                Promise.all(css.map(fetchStyleSheet)),
+              ] as const)
+            })
+            .then((res) => {
+              return this.whenEntrypoint(route).then((entrypoint) => ({
+                entrypoint,
+                styles: res[1],
+              }))
+            }),
           MS_MAX_IDLE_DELAY,
           markAssetError(new Error(`Route did not complete loading: ${route}`))
         )
@@ -421,6 +417,7 @@ export function createRouteLoader(assetPrefix: string): RouteLoader {
             }
             return { error: err }
           })
+          .finally(() => devBuildPromiseResolve?.())
       })
     },
     prefetch(route: string): Promise<void> {