WIP [esnext] implement async iteration proposal

src/heap-symbols.h

Index: src/heap-symbols.h
diff --git a/src/heap-symbols.h b/src/heap-symbols.h
index bdb0a21507d346e60c92b8a17e418dff93a27ff6..3b373c8b12a583430164d85fcbc5f9e00dc9b6b0 100644
--- a/src/heap-symbols.h
+++ b/src/heap-symbols.h
@@ -135,6 +135,7 @@
   V(RegExp_string, "RegExp")                                       \
   V(reject_string, "reject")                                       \
   V(resolve_string, "resolve")                                     \
+  V(return_string, "return")                                       \
   V(script_string, "script")                                       \
   V(second_string, "second")                                       \
   V(setPrototypeOf_string, "setPrototypeOf")                       \
@@ -232,7 +233,8 @@
   V(species_symbol, Symbol.species)          \
   V(split_symbol, Symbol.split)              \
   V(to_primitive_symbol, Symbol.toPrimitive) \
-  V(unscopables_symbol, Symbol.unscopables)
+  V(unscopables_symbol, Symbol.unscopables)  \
+  V(async_iterator_symbol, Symbol.asyncIterator)
 
 // Well-Known Symbols are "Public" symbols, which have a bit set which causes
 // them to produce an undefined value when a load results in a failed access