[promises] Add %WaitForPromise runtime call to allow tests to reliably wait for promises to be fini…

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index c94ace2dd612dcf20c5849b05746a2440924a5f8..d705f40fbbfd12026d17a169bbebcd33c7f93863 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1243,6 +1243,11 @@ class Isolate {
   // reset to nullptr.
   void UnregisterFromReleaseAtTeardown(ManagedObjectFinalizer** finalizer_ptr);
 
+  // Can be used by tests to force d8 to wait for certain things to run.
+  inline void IncrementWaitCount() { wait_count_++; }
+  inline void DecrementWaitCount() { wait_count_--; }
+  inline int GetWaitCount() { return wait_count_; }
+
  protected:
   explicit Isolate(bool enable_serializer);
   bool IsArrayOrObjectPrototype(Object* object);
@@ -1528,6 +1533,8 @@ class Isolate {
 
   size_t total_regexp_code_generated_;
 
+  int wait_count_ = 0;
+
   friend class ExecutionAccess;
   friend class HandleScopeImplementer;
   friend class HeapTester;