Use the lowest MacOS SDK available on a system

tools/gyp/all.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.
 
 # A set of variables needed to build some of the Chrome based subparts of the
 # Dart project (e.g. V8). This is in no way a complete list of variables being
 # defined by Chrome, but just the minimally needed subset.
 {
   'variables': {
     'library': 'static_library',
     'component': 'static_library',
     'target_arch': 'ia32',
     'v8_location': '<(DEPTH)/third_party/v8',
     # Flag that tells us whether to build native support for dart:io.
     'dart_io_support': 1,
   },
   'conditions': [
     [ 'OS=="linux"', {
       'target_defaults': {
         'ldflags': [ '-pthread', ],
       },
     }],
     [ 'OS=="win"', {
       'target_defaults': {
         'msvs_cygwin_dirs': ['<(DEPTH)/third_party/cygwin'],
       },
+      'includes': [
+        'msvs.gypi',
+      ],
+    }],
+    [ 'OS=="mac"', {
+      'includes': [
+        'xcode.gypi',
+      ],
     }],
   ],
   'includes': [
-    'xcode.gypi',
-    'msvs.gypi',
     'configurations.gypi',
   ],
 }