Fix Mac build.

tools/gyp/configurations_xcode.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_Base': {
         'xcode_settings': {
           # To switch to the LLVM based backend change the two lines below.
@@ skipping 30 matching lines @@
           'GCC_CW_ASM_SYNTAX': 'NO', # Remove -fasm-blocks.
 
           'GCC_ENABLE_PASCAL_STRINGS': 'NO',
           'GCC_ENABLE_TRIGRAPHS': 'NO',
           'PREBINDING': 'NO',
         },
       },
 
       'Dart_ia32_Base': {
         'xcode_settings': {
-          'ARCHS': 'i386',
+          'ARCHS': [ 'i386' ],
         },
       },
 
       'Dart_x64_Base': {
         'xcode_settings': {
-          'ARCHS': 'x86_64',
+          'ARCHS': [ 'x86_64' ],
         },
       },
 
       'Dart_simarm_Base': {
         'xcode_settings': {
-          'ARCHS': 'i386',
+          'ARCHS': [ 'i386' ],
           'GCC_OPTIMIZATION_LEVEL': '3',
         },
       },
 
       'Dart_Debug': {
         'xcode_settings': {
           'GCC_OPTIMIZATION_LEVEL': '2',
         },
       },
 
       'Dart_Release': {
         'xcode_settings': {
           'GCC_OPTIMIZATION_LEVEL': '3',
         },
       },
     },
   },
 }