WIP [esnext] implement async iteration proposal

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 795ba912fad7c0b38ec8a3e436f45b06cb7928b9..c04c5fa915f0dcacf0ff1ad3f7bff1b4bf8f9b0f 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -1237,10 +1237,14 @@ class MacroAssembler: public Assembler {
   // enabled via --debug-code.
   void AssertBoundFunction(Register object);
 
-  // Abort execution if argument is not a JSGeneratorObject,
-  // enabled via --debug-code.
+  // Abort execution if argument is not a JSGeneratorObject or a
+  // JSAsyncGeneratorObject, enabled via --debug-code.
   void AssertGeneratorObject(Register object);
 
+  // Abort execution if argument is not a JSAsyncGeneratorObject, enabled via
+  // --debug-code.
+  void AssertAsyncGeneratorObject(Register object);
+
   // Abort execution if argument is not a JSReceiver, enabled via --debug-code.
   void AssertReceiver(Register object);