[vm] Delay completing deferredPrefix.loadLibrary even if the prefix is already loaded.

c5a94db unintentionally changed the behavior of loadLibrary on a prefix that was already loaded from completing in a future turn of the message loop to completing in the same turn. Completing in the same turn is a legal behavior for loadLibrary, but consistently completing in a later turn helps to prevent code that works on accident.

Bug: #41974
Change-Id: Ib18d16f6b5517c882248f39b486e58e2635f24a1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155101
Reviewed-by: Alexander Markov <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

sdk/lib/_internal/vm/lib/lib_prefix.dart

@@ -58,13 +58,13 @@ Future<void> _loadLibrary(_LibraryPrefix prefix) async {
       }
       await load.future;
     }
-    // Ensure the prefix's future does not complete until the next Turn even
-    // when loading is a no-op or synchronous. Helps applications avoid writing
-    // code that only works when loading isn't really deferred.
-    await new Future<void>(() {
-      prefix._setLoaded();
-    });
   }
+  // Ensure the prefix's future does not complete until the next Turn even
+  // when loading is a no-op or synchronous. Helps applications avoid writing
+  // code that only works when loading isn't really deferred.
+  await new Future<void>(() {
+    prefix._setLoaded();
+  });
 }
 
 @pragma("vm:entry-point")