This adds new methods to String object:

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 155 matching lines @@
             "enable harmony object observation (implies harmony collections")
 DEFINE_bool(harmony_typed_arrays, false,
             "enable harmony typed arrays")
 DEFINE_bool(harmony_array_buffer, false,
             "enable harmony array buffer")
 DEFINE_implication(harmony_typed_arrays, harmony_array_buffer)
 DEFINE_bool(harmony_generators, false, "enable harmony generators")
 DEFINE_bool(harmony_iteration, false, "enable harmony iteration (for-of)")
 DEFINE_bool(harmony_numeric_literals, false,
             "enable harmony numeric literals (0o77, 0b11)")
+DEFINE_bool(harmony_strings, false, "enable harmony string")
 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, harmony_generators)
 DEFINE_implication(harmony, harmony_iteration)
 DEFINE_implication(harmony, harmony_numeric_literals)
+DEFINE_implication(harmony, harmony_strings)
 DEFINE_implication(harmony_modules, harmony_scoping)
 DEFINE_implication(harmony_observation, harmony_collections)
 // TODO[dslomov] add harmony => harmony_typed_arrays
 
 // 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_keyed_stores, true, "use optimizing compiler to "
             "generate keyed store stubs")
 DEFINE_bool(clever_optimizations,
@@ skipping 615 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