Attempt to fix dartium mac build

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

 # Copyright (c) 2012, 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.
 
 {
   'variables': {
     'dart_debug_optimization_level%': '2',
   },
 
   'target_defaults': {
     'configurations': {
 
       'Dart_Base': {
+        'abstract': 1,
         'xcode_settings': {
           'GCC_WARN_HIDDEN_VIRTUAL_FUNCTIONS': 'YES', # -Woverloaded-virtual
         },
       },
 
       'Dart_Debug': {
         'abstract': 1,
         'defines': [
           'DEBUG',
         ],
         'xcode_settings': {
           'GCC_OPTIMIZATION_LEVEL': '<(dart_debug_optimization_level)',
         },
       },
 
       'Debug': {
         'defines': [
           'DEBUG',
         ],
       },
 
       'Dart_ia32_Base': {
+        'abstract': 1,
         'xcode_settings': {
           'ARCHS': [ 'i386' ],
         },
       },
 
       'Dart_x64_Base': {
+        'abstract': 1,
         'xcode_settings': {
           'ARCHS': [ 'x86_64' ],
         },
       },
 
       'Dart_simarm_Base': {
+        'abstract': 1,
         'xcode_settings': {
           'ARCHS': [ 'i386' ],
           'GCC_OPTIMIZATION_LEVEL': '3',
         },
       },
 
       'Dart_Release': {
+        'abstract': 1,
         'xcode_settings': {
           'GCC_OPTIMIZATION_LEVEL': '3',
         },
       },
     },
   },
 }