[esnext] prototype runtime implementation for async functions

src/parsing/preparser.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <cmath>
 
 #include "src/allocation.h"
 #include "src/base/logging.h"
 #include "src/conversions-inl.h"
 #include "src/conversions.h"
@@ skipping 1179 matching lines @@
   {
     BlockState block_state(&scope_, block_scope);
     while (peek() != Token::RBRACE) {
       ParseStatementListItem(CHECK_OK);
     }
     Expect(Token::RBRACE, CHECK_OK);
     return PreParserExpression::Default();
   }
 }
 
+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
 
 
 }  // namespace internal
 }  // namespace v8