ES6 symbols: Implement Symbol intrinsic and basic functionality

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 123 matching lines @@
 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(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_proxies, false, "enable harmony proxies")
 DEFINE_bool(harmony_collections, false,
             "enable harmony collections (sets, maps, and weak maps)")
 DEFINE_bool(harmony_observation, false,
             "enable harmony object observation (implies harmony collections")
 DEFINE_bool(harmony, false, "enable all harmony features (except typeof)")
 DEFINE_implication(harmony, harmony_scoping)
 DEFINE_implication(harmony, harmony_modules)
+DEFINE_implication(harmony, harmony_symbols)
 DEFINE_implication(harmony, harmony_proxies)
 DEFINE_implication(harmony, harmony_collections)
 DEFINE_implication(harmony, harmony_observation)
 DEFINE_implication(harmony_modules, harmony_scoping)
 DEFINE_implication(harmony_observation, harmony_collections)
 
 // Flags for experimental implementation features.
 DEFINE_bool(packed_arrays, true, "optimizes arrays that have no holes")
 DEFINE_bool(smi_only_arrays, true, "tracks arrays with only smi values")
 DEFINE_bool(compiled_transitions, false, "use optimizing compiler to "
@@ skipping 571 matching lines @@
 #undef DEFINE_bool
 #undef DEFINE_int
 #undef DEFINE_string
 #undef DEFINE_implication
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS