[runtime] Return loading promise from _loadLocale when it already e…

…xists.

This caused things like `waitForAllDefaultIntlMessagesLoaded` and `loadAllMessagesInLocale` to return true _immediately_ if the locale was _already_ in progress loading, because it wouldn't return the same loading promise, but rather a new one that resolves as soon as the function returns.

Now they should properly wait. To ensure this still escapes early when the locale is already loaded, the existence check has been moved to the top as well.

packages/intl/src/message-loader.ts

@@ -205,8 +205,12 @@ export class MessageLoader {
   }
 
   async _loadLocale(locale: LocaleId) {
+    // If the locale is already set in `messages`, then it doesn't need to be loaded again.
+    if (this.messages[locale] != null) return;
+
     // Don't re-load a locale that's already in progress.
     if (this._localeLoadingPromises[locale]?.current != null) {
+      await this._localeLoadingPromises[locale]?.current;
       return;
     }
 
@@ -225,9 +229,6 @@ export class MessageLoader {
       }
     }
 
-    // If the locale is already set in `messages`, then it doesn't need to be loaded again.
-    if (this.messages[locale] != null) return;
-
     const current = this.localeImportMap[locale]();
     this._localeLoadingPromises[locale] = { initialized: false, current };
     this.messages[locale] = (await current).default;