[esnext] prototype runtime implementation for async functions

src/d8.h

Index: src/d8.h
diff --git a/src/d8.h b/src/d8.h
index e51e8ee13fae98b3bcc7b3d3fab5421548213454..f740b647136d9961ba69f66841fa696c1567a83c 100644
--- a/src/d8.h
+++ b/src/d8.h
@@ -401,6 +401,10 @@ class Shell : public i::AllStatic {
       const v8::FunctionCallbackInfo<v8::Value>& args);
   static void WorkerGetMessage(const v8::FunctionCallbackInfo<v8::Value>& args);
   static void WorkerTerminate(const v8::FunctionCallbackInfo<v8::Value>& args);
+
+  // Run queued microtasks and clear task queue
+  static void FlushMicrotasks(const v8::FunctionCallbackInfo<v8::Value>& args);
+
   // The OS object on the global object contains methods for performing
   // operating system calls:
   //