[esnext] implement frontend changes for async/await proposal

src/ast/ast-value-factory.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 224 matching lines @@
 
   // Internalized value (empty before internalized).
   Handle<Object> value_;
 };
 
 
 // For generating constants.
 #define STRING_CONSTANTS(F)                     \
   F(anonymous_function, "(anonymous function)") \
   F(arguments, "arguments")                     \
+  F(async, "async")                             \
+  F(await, "await")                             \
   F(constructor, "constructor")                 \
   F(default, "default")                         \
   F(done, "done")                               \
   F(dot, ".")                                   \
   F(dot_for, ".for")                            \
   F(dot_generator, ".generator")                \
   F(dot_generator_object, ".generator_object")  \
   F(dot_iterator, ".iterator")                  \
   F(dot_result, ".result")                      \
   F(dot_switch_tag, ".switch_tag")              \
@@ skipping 113 matching lines @@
   OTHER_CONSTANTS(F)
 #undef F
 };
 }  // namespace internal
 }  // namespace v8
 
 #undef STRING_CONSTANTS
 #undef OTHER_CONSTANTS
 
 #endif  // V8_AST_AST_VALUE_FACTORY_H_