Fix a potential race condition in router when doing prefetching. (#2317)

lib/page-loader.js

@@ -30,17 +30,10 @@ export default class PageLoader {
   loadPage (route) {
     route = this.normalizeRoute(route)
 
-    const cachedPage = this.pageCache[route]
-    if (cachedPage) {
-      return new Promise((resolve, reject) => {
-        if (cachedPage.error) return reject(cachedPage.error)
-        return resolve(cachedPage.page)
-      })
-    }
-
     return new Promise((resolve, reject) => {
       const fire = ({ error, page }) => {
         this.pageRegisterEvents.off(route, fire)
+        delete this.loadingRoutes[route]
 
         if (error) {
           reject(error)
@@ -49,6 +42,15 @@ export default class PageLoader {
         }
       }
 
+      // If there's a cached version of the page, let's use it.
+      const cachedPage = this.pageCache[route]
+      if (cachedPage) {
+        const { error, page } = cachedPage
+        error ? reject(error) : resolve(page)
+        return
+      }
+
+      // Register a listener to get the page
       this.pageRegisterEvents.on(route, fire)
 
       // If the page is loading via SSR, we need to wait for it