WIP [esnext] implement async iteration proposal

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index 3f3ecce134895b3005d15b45ca2d4f69c568b5db..763906acc17dbcd84368cfb1783141c9b0b938f7 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -473,6 +473,8 @@ class ErrorUtils : public AllStatic {
   T(StrictCannotCreateProperty, "Cannot create property '%' on % '%'")         \
   T(SymbolIteratorInvalid,                                                     \
     "Result of the Symbol.iterator method is not an object")                   \
+  T(SymbolAsyncIteratorInvalid,                                                \
+    "Result of the Symbol.asyncIterator method is not an object")              \
   T(SymbolKeyFor, "% is not a symbol")                                         \
   T(SymbolToNumber, "Cannot convert a Symbol value to a number")               \
   T(SymbolToString, "Cannot convert a Symbol value to a string")               \