Create the SDK in a temporary directory and copy it as a final step.

dart.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.
 
 {
   'targets': [
     {
       'target_name': 'compiler',
       'type': 'none',
       'dependencies': [
@@ skipping 36 matching lines @@
           'action_name': 'create_sdk_py',
           'inputs': [
             'tools/create_sdk.py',
           ],
           'outputs': [
             '<(PRODUCT_DIR)/sdk',
           ],
           'action': [
             'python',
             'tools/create_sdk.py',
-            '<(PRODUCT_DIR)/sdk'
+            '<(PRODUCT_DIR)/sdk',
           ],
           'message': 'Creating SDK.',
         },
       ],
     },
     {
       'target_name': 'upload_sdk',
       'type': 'none',
       'dependencies': [
         'create_sdk',
@@ skipping 20 matching lines @@
     # so don't build it for now.
     #{
     #  'target_name': 'client',
     #  'type': 'none',
     #  'dependencies': [
     #    'client/dart.gyp:fling',
     #  ],
     #},
   ],
 }