Move throttling

lib/src/back_end/worker_pool.dart

diff --git a/lib/src/back_end/worker_pool.dart b/lib/src/back_end/worker_pool.dart
index 63415e6..4e250af 100644
--- a/lib/src/back_end/worker_pool.dart
+++ b/lib/src/back_end/worker_pool.dart
@@ -79,11 +79,6 @@
   }) async {
     if (_isClosed) throw StateError('WorkerPool is closed');
 
-    if (_inMemoryCount >= maxBacklog) {
-      _throttleCompleter ??= Completer<void>();
-      await _throttleCompleter!.future;
-    }
-
     _inMemoryCount++;
 
     var request = _WorkerRequest(
@@ -110,6 +105,11 @@
     if (_pending.length >= _batchSize) {
       _flush();
     }
+
+    if (_inMemoryCount >= maxBacklog) {
+      _throttleCompleter ??= Completer<void>();
+      return _throttleCompleter!.future;
+    }
   }
 
   /// Sends a batch of requests to a worker.