[es7] Implement async functions parsing

src/js/v8natives.js

 // Copyright 2012 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.
 
 (function(global, utils) {
 
 %CheckIsBootstrapping();
 
 // ----------------------------------------------------------------------------
 // Imports
@@ skipping 1665 matching lines @@
 
   if (%FunctionIsArrow(func)) {
     return source;
   }
 
   var name = %FunctionNameShouldPrintAsAnonymous(func)
       ? 'anonymous'
       : %FunctionGetName(func);
 
   var isGenerator = %FunctionIsGenerator(func);
+  var isAsync = %FunctionIsAsync(func);
   var head = %FunctionIsConciseMethod(func)
       ? (isGenerator ? '*' : '')
       : (isGenerator ? 'function* ' : 'function ');
-  return head + name + source;
+  return (isAsync ? 'async ' : '') + head + name + source;
 }
 
 
 function FunctionToString() {
   return FunctionSourceString(this);
 }
 
 
 // ES5 15.3.4.5
 function FunctionBind(this_arg) { // Length is 1.
@@ skipping 149 matching lines @@
 %InstallToContext([
   "global_eval_fun", GlobalEval,
   "object_value_of", ObjectValueOf,
   "object_to_string", ObjectToString,
   "object_define_own_property", DefineOwnPropertyFromAPI,
   "object_get_own_property_descriptor", ObjectGetOwnPropertyDescriptor,
   "to_complete_property_descriptor", ToCompletePropertyDescriptor,
 ]);
 
 })