This adds new methods to String object:

tools/gyp/v8.gyp

 # 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 886 matching lines @@
         ],
         'experimental_library_files': [
           '../../src/macros.py',
           '../../src/symbol.js',
           '../../src/proxy.js',
           '../../src/collection.js',
           '../../src/object-observe.js',
           '../../src/arraybuffer.js',
           '../../src/typedarray.js',
           '../../src/generator.js',
-          '../../src/array-iterator.js'
+          '../../src/array-iterator.js',
+          '../../src/harmony-string.js'
         ],
         'i18n_library_files': [
           '../../src/extensions/i18n/header.js',
           '../../src/extensions/i18n/globals.js',
           '../../src/extensions/i18n/locale.js',
           '../../src/extensions/i18n/collator.js',
           '../../src/extensions/i18n/number-format.js',
           '../../src/extensions/i18n/date-format.js',
           '../../src/extensions/i18n/break-iterator.js',
           '../../src/extensions/i18n/i18n-utils.js',
@@ skipping 115 matching lines @@
         }],
         ['v8_compress_startup_data=="bz2"', {
           'libraries': [
             '-lbz2',
           ]
         }],
       ],
     },
   ],
 }