[esnext] prototype runtime implementation for async functions

src/parsing/preparser.cc

Index: src/parsing/preparser.cc
diff --git a/src/parsing/preparser.cc b/src/parsing/preparser.cc
index c3f6802f2e8c4f880eecefcd0488a3e8a9817743..7515a3a4f522928463c37329e3ea6bd1e37095ff 100644
--- a/src/parsing/preparser.cc
+++ b/src/parsing/preparser.cc
@@ -1118,6 +1118,7 @@ PreParser::Expression PreParser::ParseFunctionLiteral(
   bool is_lazily_parsed = (outer_is_script_scope && allow_lazy() &&
                            !function_state_->this_function_is_parenthesized());
 
+  function_state.set_parse_phase(FunctionParsePhase::FunctionBody);
   Expect(Token::LBRACE, CHECK_OK);
   function_state.set_parse_phase(FunctionParsePhase::FunctionBody);
   if (is_lazily_parsed) {
@@ -1292,6 +1293,19 @@ PreParserExpression PreParser::ParseDoExpression(bool* ok) {
   return PreParserExpression::Default();
 }
 
+void PreParserTraits::ParseAsyncArrowSingleExpressionBody(
+    PreParserStatementList body, bool accept_IN,
+    Type::ExpressionClassifier* classifier, int pos, bool* ok) {
+  Scope* scope = pre_parser_->scope_;
+  scope->ForceContextAllocation();
+
+  PreParserExpression return_value =
+      pre_parser_->ParseAssignmentExpression(accept_IN, classifier, ok);
+  if (!*ok) return;
+
+  body->Add(PreParserStatement::ExpressionStatement(return_value), zone());
+}
+
 #undef CHECK_OK