Use the lowest MacOS SDK available on a system

compiler/dartium.gyp

 # 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.
 
 # TODO(vsm): Remove this file and use dart.gyp once that can be pulled
 # into the dartium build.
 {
   'includes': [
-    # TODO(iposva): Move shared gyp setup to a shared location.
-    '../tools/gyp/xcode.gypi',
     # TODO(mmendez): Add the appropriate gypi includes here.
   ],
   'targets': [
     {
       'target_name': 'dartc',
       'type': 'none',
       'actions': [
         {
           'action_name': 'Build and test',
           'inputs': [
@@ skipping 11 matching lines @@
         },
       ],
     },
     {
       # GYP won't generate a catch-all target if there's only one target.
       'target_name': 'dummy',
       'type': 'none',
     },
   ],
 }