Ensure all debug messages are flushed.

dart/runtime/dart-runtime.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.
 
 {
   'includes': [
     'vm/vm.gypi',
     'bin/bin.gypi',
     'third_party/double-conversion/src/double-conversion.gypi',
     'third_party/jscre/jscre.gypi',
@@ skipping 73 matching lines @@
           'inputs': [
             'tools/make_version.py',
             '../tools/VERSION',
             '<(version_in_cc_file)',
           ],
           'outputs': [
             '<(version_cc_file)',
           ],
           'action': [
             'python',
+            '-u', # Make standard I/O unbuffered.
             'tools/make_version.py',
             '--output', '<(version_cc_file)',
             '--input', '<(version_in_cc_file)',
             '--version', '../tools/VERSION',
           ],
         },
       ],
     },
   ],
 }