[esnext] Async function toString support

src/objects.cc

 // Copyright 2015 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 "src/objects.h"
 
 #include <cmath>
 #include <iomanip>
 #include <sstream>
 
@@ skipping 12174 matching lines @@
   }
 
   // Check if we have source code for the {function}.
   if (!shared_info->HasSourceCode()) {
     return NativeCodeFunctionSourceString(shared_info);
   }
 
   IncrementalStringBuilder builder(isolate);
   if (!shared_info->is_arrow()) {
     if (shared_info->is_concise_method()) {
-      if (shared_info->is_generator()) builder.AppendCharacter('*');
+      if (shared_info->is_generator()) {
+        builder.AppendCharacter('*');
+      } else if (shared_info->is_async()) {
+        builder.AppendCString("async ");
+      }
     } else {
       if (shared_info->is_generator()) {
         builder.AppendCString("function* ");
+      } else if (shared_info->is_async()) {
+        builder.AppendCString("async function ");
       } else {
         builder.AppendCString("function ");
       }
     }
     if (shared_info->name_should_print_as_anonymous()) {
       builder.AppendCString("anonymous");
     } else if (!shared_info->is_anonymous_expression()) {
       builder.AppendString(handle(String::cast(shared_info->name()), isolate));
     }
   }
@@ skipping 6084 matching lines @@
   if (cell->value() != *new_value) {
     cell->set_value(*new_value);
     Isolate* isolate = cell->GetIsolate();
     cell->dependent_code()->DeoptimizeDependentCodeGroup(
         isolate, DependentCode::kPropertyCellChangedGroup);
   }
 }
 
 }  // namespace internal
 }  // namespace v8