Add --es-staging flag

src/flag-definitions.h

 // Copyright 2012 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 149 matching lines @@
   FLAG_ALIAS(STRING, const char*, alias, nam)
 #define DEFINE_ALIAS_args(alias, nam)  FLAG_ALIAS(ARGS, JSArguments, alias, nam)
 
 //
 // Flags in all modes.
 //
 #define FLAG FLAG_FULL
 
 // Flags for language modes and experimental language features.
 DEFINE_bool(use_strict, false, "enforce strict mode")
-DEFINE_bool(es5_readonly, true,
-            "activate correct semantics for inheriting readonliness")
-DEFINE_bool(es52_globals, true,
-            "activate new semantics for global var declarations")
+DEFINE_bool(es_staging, false, "enable upcoming ES6+ features")
 
 DEFINE_bool(harmony_typeof, false, "enable harmony semantics for typeof")
 DEFINE_bool(harmony_scoping, false, "enable harmony block scoping")
 DEFINE_bool(harmony_modules, false,
             "enable harmony modules (implies block scoping)")
 DEFINE_bool(harmony_symbols, false,
             "enable harmony symbols (a.k.a. private names)")
 DEFINE_bool(harmony_promises, false, "enable harmony promises")
 DEFINE_bool(harmony_proxies, false, "enable harmony proxies")
 DEFINE_bool(harmony_collections, false,
@@ skipping 731 matching lines @@
 #undef DEFINE_ALIAS_float
 #undef DEFINE_ALIAS_args
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA