[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 27c6d23bbee171a2a0c13569fe36f726615acbae..9790ac693a6e1a2a706276290450411508d821ec 100644
--- a/src/parsing/preparser.cc
+++ b/src/parsing/preparser.cc
@@ -1197,6 +1197,36 @@ PreParserExpression PreParser::ParseDoExpression(bool* ok) {
   }
 }
 
+void PreParserTraits::ParseAsyncArrowSingleExpressionBody(
+    PreParserStatementList body, bool accept_IN,
+    Type::ExpressionClassifier* classifier, int pos, bool* ok) {
+  {
+    Scope* generator_scope = pre_parser_->NewScope(
+        pre_parser_->scope_, FUNCTION_SCOPE, kAsyncFunction);
+
+    // For generators, allocating variables in contexts is currently a win
+    // because it minimizes the work needed to suspend and resume an
+    // activation.  The machine code produced for generators (by
+    // full-codegen)
+    // relies on this forced context allocation, but not in an essential
+    // way.
+    generator_scope->ForceContextAllocation();
+
+    PreParser::FunctionState generator_state(
+        &pre_parser_->function_state_, &pre_parser_->scope_, generator_scope,
+        kAsyncFunction, pre_parser_->factory());
+    generator_scope->set_start_position(pos);
+
+    PreParserExpression return_value =
+        pre_parser_->ParseAssignmentExpression(accept_IN, classifier, ok);
+    if (!*ok) return;
+
+    generator_scope->set_end_position(
+        pre_parser_->scanner()->location().end_pos);
+    body->Add(PreParserStatement::ExpressionStatement(return_value), zone());
+  }
+}
+
 #undef CHECK_OK