Remove --promise-extra flag

src/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 2177 matching lines @@
           'debug/mirrors.js',
           'debug/debug.js',
           'debug/liveedit.js',
         ],
         'experimental_library_files': [
           'js/macros.py',
           'messages.h',
           'js/harmony-atomics.js',
           'js/harmony-simd.js',
           'js/harmony-string-padding.js',
-          'js/promise-extra.js',
           'js/harmony-async-await.js'
         ],
         'libraries_bin_file': '<(SHARED_INTERMEDIATE_DIR)/libraries.bin',
         'libraries_experimental_bin_file': '<(SHARED_INTERMEDIATE_DIR)/libraries-experimental.bin',
         'libraries_extras_bin_file': '<(SHARED_INTERMEDIATE_DIR)/libraries-extras.bin',
         'libraries_experimental_extras_bin_file': '<(SHARED_INTERMEDIATE_DIR)/libraries-experimental-extras.bin',
         'conditions': [
           ['v8_enable_i18n_support==1', {
             'library_files': ['js/i18n.js'],
             'experimental_library_files': [
@@ skipping 205 matching lines @@
       'conditions': [
         ['want_separate_host_toolset_mkpeephole==1', {
           'toolsets': ['host'],
         }, {
           'toolsets': ['target'],
         }],
       ],
     },
   ],
 }