Update V8 to 3.12.

dart/runtime/tools/gyp/runtime-configurations.gypi

 # Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 {
   'target_defaults': {
     'configurations': {
       'Dart_Debug': {
         'abstract': 1,
         'defines': [
           'DEBUG',
         ],
       },
+
       'Debug': {
         'defines': [
           'DEBUG',
         ],
       },
+
+      'Dart_ia32_Base': {
+        'xcode_settings': {
+          'ARCHS': [ 'i386' ],
+        },
+      },
+
+      'Dart_x64_Base': {
+        'xcode_settings': {
+          'ARCHS': [ 'x86_64' ],
+        },
+      },
     },
   },
 }